Usmash is junk; utilt has all of the vertical range that usmash does (and not much less horizontal range), it launches farther, it can be sweetspotted, and since it's a one-hit move, it can't be DI'ed out of. Oh, and it has less lag and it does around the same percentage as a Usmash that connects with all three hits. So yeah, usmash is just a bad habit (that I have :\ )
Fsmash has its uses, though...after all, it is actually his most powerful smash. From the time when you press fsmash on the controller to the time when it comes out, there is a good amount of lag; however, if you charge fsmash, there isn't much lag at all after the charge release, so it can be used when edgeguarding or during some other time when you have the time to wind up an fsmash. I guess you could also use the fsmash's windup time to catch your opponent off-guard or as some form of a mind game...?
![Stick Out Tongue :p :p](/styles/default/xenforo/smilies/tongue.gif)
While it is true that you should dsmash as your normal kill move and as a punishing option on the ground, fsmash has its uses.