I think that's a pretty good summation, Apathy. Personally, I don't mind them being powerful, I much prefer the role play to the roll play but I felt a slowly accruing system of drain would help balance it a little. If you cast 3 force 6 spells (assuming a magic 6 with no desire to overcast) that would add 2 to all subsequent drain codes until sunrise/set. I personally didn't feel that would affect most mages, as the ones I have partied with start the fight with a powerful whack and it's usually just mopping up from there.
I have to agree that this isn't such a great route in my opinion.
However, if you DO want to increase drain as time goes by I would suggest coming at it from the opposite angle and instead of increasing the drain DV, reduce the DP of the drain resistance pool.
Since it's average of 3 dice to get a consistent hit, adding 1 DV of damage means you'd need another 3 die to cover it. Then add another DV, etc. You'll be taking very large amounts of drain quickly.
Conversely, if you were to reduce the DP of the drain resistance pool by 1 in the same manner as you suggested, the mage would sill be more likely to take increases in drain but it wouldn't be as fast or as guaranteed.
I would argue that it would fit fluff better too, as tiring doesn't make the task any harder, it reduces the character's ability to achieve the same results on the same level of difficulty.
But once again, I don't really think this is the way to do it, just a kinder and more consistant version if you feel you must.