no need to justify the need. its cool to play with in general lol
it all comes down to what you are trying to do really, and your price range.
are you looking for it to come with preloaded sounds? (not necessarry and i would advise against it. there are literally thousands of virtual instruments that you can get for free or torrent.)
as for pure controllers, if you're looking to spend as little money as possible, look at the alesis series
http://www.musiciansfriend.com/keybo...board&index=14
i've got the 25 in my closet. since i got the mpc i don't use it, and i never really got the hang of playing keys enough for them to be in my workflow. still cool to play with though.
also m-audio is decent, the only reason i recommend alesis is its the same shit. neither have weighted keys, both function exactly the same, the alesis is just cheaper in each tier, usually. (25, 49, 61, 88) i've had m-audio controllers and alesis. the only two i've had.
now if you know how to play keys, like in the both hands sense i'd recommend at least a 61 key model. the 49 is serviceable as well for alot of stuff but if you want the piano experience the more the better. i have the 25 now because i'm a one chord at a time + key poker type dude, i don't need to be playing chords with one hand WHILE poking. all these keyboards will come with the octave switch thing, so the 25, represents 25 keys at a time, you hit the + sign and it makes it the next 25 keys and so on.
now if you're willing to drop more than 200 and want it to be DOPE
you gotta look into weighted keys. at least semi weighted make a huge difference, espescially when playing rhodes sounds and organs and shit. with pianos you can find the velocities easier but if you want real milky smooth shit your gonna need real feel keys. well not NEED but it makes it easier.
as far as that goes i have no personal experience outside of messing with them at guitar center lol. so i can't really help there
but yea
basically just make sure what you get has full size keys, semi weighted and fully weighted keys are better than regular keyboard keys but also those keyboards are more expensive, and don't worry about it coming with sounds. i can point you in the direction of more sounds than you will ever use at a professional level