I have 10 2.5g tanks and i have filters in none of them. In fact, I have several whispers here that i'll never use.
I do 100% water changes on all of them about every 4 days. It gets to be no big deal after a while - in fact they all rather enjoy it, and I rearrange the stuff in their tanks w/every change so it's fun for them when they go back in and explore their new turf...
2 of the tanks are at work, the rest are all here @ home. They all have rocks, plants and caves. I take out the decor, rinse the gravel, put the water back in, rearrange and viola. I get the 8 I have here at home done in less than an hour. The 2 I have at work I do over lunch on Mondays and Thursdays.
Every time I go into PetWorld and see the minibow they have set up right up front with the filter system in it that comes w/the tank I cringe. I was just there tonight and this poor betta was pressing himself up against the big rock cave that was in his tank so he could rest w/out being blown all over. He was very clamped up and obviously exhausted from fighting the flow.
In my opinion, gravel vacuuming and water changes are much more of a hassle than 100% changes are anyway, especially in a tank that size.