I've been thinking about this for a while, something to discuss for a future edition, or a possible House Rule. It's been my experience that magi need something that limits them in some way that is meaningful, but it doesn't distract from the terrible power that they can bring to confront any particular problem. Magi can be designed to be able to cast spells without any appreciable limits, it's not difficult to design a combat caster who can subtract 6 botch dice from their casting rolls, indeed a starting magus can be designed easily to remove 4 or 5 botch dice, with 1 virtue point (cautious sorcerer) and 5-15 xp (1 or 2 ability skill in mastery).
My idea isn't true fatigue, where if you cast enough spells that cost you fatigue that you end up passing out, but it's more that, the more often you use magic in quick, rapid succession, the more risk you take in losing control over said magic, that is your botch risk increases, and that as you cast magic more and more often in succession, it automatically becomes stressful, thus tamping down on the Monte Carlo approach players often use to achieve an end with a mastered spell and rolling until one gets an explosion of sufficient magnitude to satisfy the result. Perhaps the first spell is free, whereas subsequent spells introduce the botch penalty, similar to how the fatigue mechanic works in that the first fatigue level costs nothing. Note, I'm not saying a 1, 3, 5 progression for botch dice. Just each spell cast before adds a botch die, except the first.
In my experience, combat rarely lasts more than 6 rounds, but is often more like 3, especially as magi exercise their Arts in the conflict. My idea would be to increase the botch dice for a magus as he casts a spell each round. This means that combat casters need to invest heavily in their favored combat spells, but also consider mastering their utility spells that get used in combat, too. IT means the aftermath of battle, where spells might be employed to save lives are not safe periods with which to cast spells.
I know that there will be some people who will hate this idea. If you do hate the idea, that's fine, but I'd like to see you add something constructive rather than merely saying, it's dumb and Ars Magica doesn't need this or some variant thereof. I know that there are people out there who feel similarly to me, that magic needs to have some sort of cost. I don't want to place artificial limits, such as a power point system, or limit the level of spells in some way and when you hit that, you're done. Although, perhaps it's the number of levels of spells that causes increased risk, and not the use in quick succession?