The reason I use keyboard amps rather than powered speakers is that I have the choice of which passive speakers (and which amp) I use on a gig. I'm talking here, of course, about separate amp and speakers - I stopped using keyboard combos some years ago.
Using powered speakers meant extra power cables to each speaker (something I prefer not) and if the amp in a powered speaker went down, that just wouldn't work. I now tend to use a stereo amp (150w) with two small (8") cabs - mainly for my benefit as monitors. Which can be added to for medium gigs. Bigger gigs then go through that and then the p.a. I get the benefit of stereo strings, Leslie wafting, chorus Rhodes, etc. The audience doesn't, particularly, but the band does!
As far as sound production is concerned, I don't hear much difference between the two, both will be full range, powerful enough for the job, so I guess personal choice is what it comes down to. Dedicated k'bd amps will have extra ports for fx send/return, and often a graphic eq., which are essential for some players, which won't be available on all powered speakers.