Dumb question time...
What is the point of the Wizard's Communion spell?
It doesn't let a group cast something they couldn't already, because one of the magi needs to know spell being cast. It adds extra botch die and puts everyone at risk of Twilight.
I've played ArsM for years and I have never understood the purpose of this spell.
Penetration
and possibly casting tablets?
That's it? 
Here's a question, and I generally don't know the answer...
As learning a spell is the same as inventing one, only you have a guide from a lab text or teacher, can you get assistance in the lab to increase your total? If so, you could potentially learn spells outside your normal range but have to rely on your covenant/cult for the power to cast them.
Sounds like a nice way to bind a covenant/cult together to me.
Without looking, I think it also allows one to use casting scrolls far over their (safe) ability.
Also oto vercome massive penalties, such as casting in high Dominion areas or an unfriendly Aegis, etc etc.
There may be more creative uses, esp w/ certain virtues.
Nice, those penetrations................you have to cast ceremonially, so it has no real combat potential. Except for arcane connected spells.........
That's right, in fact with a strong magic theory, intelligence, laboratory modifications, inventive genius and so on it is entirely possible for an unaided character to learn a spell that they can't cast.
Yes! That's it!!!
In our saga wards and the Aegis have to penetrate to take effect. Living at the edge of hermetic lands (Denmark) we have an Aegis level 30(!!). My character usually casts it with a casting total of about 45. So Penetration is 15 + 1 (Penetration Score 
He is a lab rat) =16.
So creatures with might 20 enter the convent unhindered.
Usind four magi the casting level dropps to 45/4=12 and Penetration is 33+1= 34! (More than doubled!)
If you want to penetrate a dragon's MR (50) and you have a final casting total of 60 (including penetration bonusses etc.), you've got two main options*:
The first option is way easier, but if you have an AC to the dragon and a group that can cast Wizards Communion, the second option gives a better result.
[size=75]* I know this is an oversimplification: but it does show a powerfull use of WC[/size]
We have the same sort of thing. We have 5 magi and a level 25 Aegis cast at about 25 + penetration(2/3). This would mean that the spell would have a very low penetration and be quite pointless as again our aegis/wards need to penetrate.
with 5 magi casting the penetration usually rises to about 25 making the aegis a lot more useful.
Wizards communion is also useful for actually casting spells which are a bit too much for wizards. A specialised person may JUST learn a spell at the edge of his capability. By having a communion going this makes the spell a lot more capable and easier to cast, even if it is slow.
Don't forget casting tablets. Independent of your saga's view on penetration of the Aegis of the Hearth, a casting tablet makes it very possible to have a spell that is beyond your capabilities to learn (perhaps significantly beyond) yet still cast it.
Dangerous? Yep. But less so if everyone has spent an extra season mastering WC.
For example, a young covenant in a dangerous location may decide to contact some crazy old wizard and offer to pay him or her to develop a spell that would take years, perhaps decades, for them to develop the capability to invent themselves. Extending the idea just a bit further, a casting tablet is a good way for a covenant to let it's magi stay focused on their particular specialties.
It also makes it possible for non-Verditius specialists to make fortunes in selling casting tablets. If your magus is a Rego Vim specialist, have him develop a 10th (!) magnitude Aegis. There are magi paranoid enough to want a copy. How about Perdo Vim? A ritual version of Winds of Mundane Silence cast at AC range with enough Penetration to get through Parma AND a moderate Aegis. Creo Herbam? A sight range version of Bountiful Harvest cast at the head of the valley your covenant of Terram magi are living/hiding. An old and focused specialist can put together a lab total of 70 in his or her specialty. Three or four or five seasons spent developing the 10th or 11th maginitude spell, another season or two making casting tablets, and a little sales work at Tribunal could yeild a substantial yearly income in vis and/or BIG favors for a several years.
What surprises me, I suppose, is that there are covenants out there that don't insist that their members know Wizards Communion because the only spells cast to aid the covenant don't require the extra power. They are walking away from great opportunities to make thier covenants wealthier, their lives safer for themselves and essential mundanes at least, and (subtly, of course) make their neighbors less inclined to threaten them.
Just a thought.
-K!
WC is way less usefull (and cool) than it was in 4th ed., that's for sure.
That was because it was possible to cast spell from books. But it was too unbalancing IMO (mages adventuring with books,etc)
Still, I think WC has it's uses in 5th ed.
Our Covenant ask of every member to know WC and to master it.
(And we have a tractatus on the subject of such a mastery, and a lab text of WC level 20.)
We use it once per year to cast the Aegis of the Hearth level 30, to get sufficient penetration. My own caracter cast it at 60 casting total, he is a Ward Specialist. Having high penetration on the Aegis is important, because even if a creature of 50 Might can enter, he will not be able hurt the mages with a penalty of 15 on his powers.
Now that our covenant gained two new members, (from 3 to 5) we'll plan to obtain a higher level WC, and cast an Aegis of level 55 (!).
The more members a covenant have, the more powerfull his Aegis will be.
I'll have to learn the rest of my covenant the wc yet. I think aegis are the best reason, you may be able to spy a magus through an aegis this way...
I have saw many times the Aegis of the hearth, and didn't say anything about Penetration required, the spell don't need be cast and known only by one participant, and other Magi and Hedge Wizard can participate simply like partners. The spell result is the Level, that must be penetrated and in that count the penetration of foreigns not invited is reduced. The Comunion only help to the group in make great the Level, but the spell isn't really a regular spell; is for say, a way for the Vis and aura are aligned to the particiapnts, and of that way is the Rego Technique and no other. Is like throw a rock with finess.
The other ways the Comunion still being necessary for Penetration and total Cast, but the Aegis isn't the primary objectiv.
Xavi agrees with Mario
We used to require wizard's communion for our magi in our previous sagas. Now we do not even have it in our library. It is useful if you have BIG casting tablets, biut we have not gone that route so far.
Cheers,
Xavi