I personally think that it's a big use of the Developer's time coding each build of FuSa for 3 different firmwares now (3.8, 3.9, 4.01), and that there are now enough reasons to upgrade to suggest that FuSa shouldn't be pressured for backwards compatibility. The following are reasons to use newer firmware (4.01+): -Skype(I use this for making my calls and such)
-PSPStates(Savestates! Save and load from any point in any PSP game, it's amazing!)
-CXMB (exists for others but the latest is for 4.01)
-Video Speed Playback (this is very fun to fool around with, I missed it when I went to 3.90 to beta test FuSa b47)
-Pressing select to switch camera modes (well not everyone has the camera but for those of us that do... O_O)
-I suppose internet search for those that use it
I just feel like there is now a good reason to update for Slim users especially. PSPStates was enough to convince me alone
Maybe if you want to still support 3.90 that would be fine but I see no reason at all to stay with 3.80... I've been on 4.01 and the listed reasons are really what's keeping me there
Just a suggestion of course 
i love 5.00 m33-3