I agree. I do a lot of 3.5 gallon (2.5 gallons into keg) batches as well and it's a nice size, especially if you don't share that much or drink that much (both of which I occasionally do). Sometimes a batch will last a week and a half in the keg, sometimes a month. That's kinda nice, easy to run out of beer fast though. But, like you said, you can brew more often, which I have lots of time to do. Bigger batches makes perfect sense for those who have lots of parties and/or can't brew that often, both of which don't apply to me.
That said, I still brew 5 gallon batches as well. But sometimes a keg will last 2 months. Funny thing is, I drink the first, about, 2.5 gallons pretty quick, then the last 2.5 gallons takes forever it seems. Even back in 2010 when I first started kegging I always felt like I got tired of a beer about halfway through the keg.
Today I'm brewing a 3.5 gallon batch of Traditional Bock that I'll lager until sometime in the late fall, I'm thinking.