Like all the best systems, there are subtleties.
Let us consider a small sample of theoretical Mages.
Magus Smartypants the Weakling with +3 Int and -3 Sta will be able to accumulate knowledge scores faster (that is Stat +ability scores relying on Int) and learn/invent more complicated things in the labs (higher level spells for example). His magic may be more developed, but have less puff behind it.
Magus BoxofHammers Builtlikeabrickshithouse with -3 Int and +3 Sta will have more oomph behind his magic - more raw power if you will, but will lack the knowledge to back that up - he can't learn/invent the same level of spells because his lab score is substantially lower.
Consider both mages with Art scores of 5 in everything and MT 4. Given a standard lab and a moderate +3 Aura, SPtW can learn level 20 spells (5+5+4+3+3) while BoHBlaBSH cannot even learn level 15 (5+5+4-3+3).
They both learn a CrIg combat spell (taking one full season).
BoHBlaBSH can learn a Lvl 10 spell with a casting total (sans aura) of 13+DR. He can cast all day no problem, and can penetrate low levels of Magic resistance.
SPtW can learn a Lvl20 spell (PoF) with a casting total (sans aura) of 7+DR. He finds casting this tiring, may even fail sometimes, and has very little chance of penetrating any MR at all. But its does a lot more damage and is a hell of a lot scarier if he gets it off.
Meanwhile Magus Talkthehindlegsoffadonkey with Int 0 Sta 0 Com +4 finds that nobody else cares about the other two Magi's work while he can trade/sell his tractati at a premium price.
And Magus Lookatmei'mawesome with his Int 0 Sta 0 Com 0 Pre +4 is developing into prime leadership material within the group and ends up being the Head Honcho of this little covenant telling all the others what to do.
There simply isn't a simple rule of thumb for mages that they all favour one particular stat more than any others. Each has their place, and Magi of all types can be found.
To suggest otherwise is using a Box of Hammers.
Corbon