I'd certainly agree that Intelligence is worthwhile, principally as you state for the benefit to Lab Total. I would say that Stamina is really important too, however.
The small bonus to your Casting Total from Stamina is OK, but not that wonderful, except in the relatively rare situation where you are casting spells that have a small chance of Penetrating. Usually, in my experience, you either cannot (or do not need to) Penetrate with your spells, or are easily able to Penetrate. However if Penetration is a bit marginal, but doable, then having an extra point or two of Stamina can really make the difference between hoping that you roll a 1 on the stress die, and having a decent chance.
Where Stamina really counts is Soak and Certamen.
Now I realize that you can cast spells that increase your Soak, and you can of course try to completely avoid situations that require you to Soak damage, but if you have a character that intends to get in the way of damage having heaps of Stamina is fantastic. Particularly for young(ish) magi who don't have the ability to cast a wide range of defensive spells either formulaic or spontaneous, particularly when standing in a hostile aura.
The place of Stamina in the Certamen Resistance Total means that a high Stamina character can be great at Certamen. Now, of course the importance of this depends on how often your characters use Certamen. For example, my magus character, in my current saga, is sort of accidentally really good at Certamen because of his high Stamina Score (he has a high Stamina Score because he is designed to beat people up). Although he is not really exceptional at any Arts, he wins most Certamen matches against his sodales and similar age magi, simply because his Resistance Total is about twice everyone elses.
So, I would say that from a pure power point of view Intelligence and Stamina are the two most useful characteristics. Which is the most useful depends on the type of character and the type of story you want your character to excel in. And, of course, it is not compulsory to have either characteristic particularly high. You can make a perfectly good magus with relatively modest characteristics, and playing a magus with bad characteristics can be heaps of fun too.
We now have six stats remaining, each of which has something important to offer. Qik is important for going first, unless you don't even know what's coming, so Per surely also matters. Int shows up in all sorts of interesting rolls and totals, as does Sta. An extra point or two on a casting total never hurts, especially if it also grants Soak. Pre is good for initiation, Com is good for writing, and both are good for socializing, which makes both Pre and Com the most valuable stats at Tribunal, where casting totals don't help much.
