I wouldn't say that practicing vs. cpus is bad per se, but they can promote
bad habits when facing real players. Surely you've noticed how they'll
never do anything other than air dodge after using it once? Or how
they never pursue you into the air after launching you up, instead just walking
to where they think you'll land? Real players don't always act like this.
Cpus may be faster than us, but they certainly aren't the smartest.
This isn't particularly true, to be a stickler. CPUs do follow with aerials in Smash 4 all of the time.
@OP
The issue with CPU characters is they never change up their game or adapt. They are basically 100% reactionary, with a very default agenda otherwise. Try starting a match and just standing there, and you'll likely witness some of the strangest behavior you'd never see from a player.
While it's not like defensive acting players don't exist, what pushes the issue further is that they generally do not adapt or attempt to outplay you. You can repeat the same sure fire combo/technique ad nauseam and they will never be the wiser, especially when it comes to being off the edge. Many players like to be extremely aggressive, and CPUs generally across the board do not prepare you for that type of player behavior.
Now, does that make CPUs
bad to practice with? Not at all, but at the end of the day if you had to make the comparison, a lvl 9 character could be equated to a player at a pretty low end of the average spectrum, so you're not really prepping yourself for much, and it makes a lot less sense if you have the ability to fight online in continuous battles with other real players (given you're not plagued with a terrible connection or anything else that leads to you online experience not being optimal).
CPU's essentially do not prepare you to think for yourself, they train you to develop a habitual approach that in the end will fail you against a player using a real brain. So it's not necessarily that they are bad to train with, but it is bad to treat them like a real player and use that to gauge your skill. They don't cover all the truly necessary bases for fighting against humans. Because of this, I recommend when fighting CPUs to ignore the part about
beating them, and just go for combos. You know that if what you're doing is escapable, a 9 will escape it (better than anyone). So use them sort of as punching bags, per se. Just as you wouldn't consider a punching bag a formidable boxing opponent, you shouldn't also consider a lvl 9 CPU a person. You wouldn't crush a punching bags stuffing out and then think you're ready to box with a real guy, right? Don't aim to beat them. Aim to practice your skills on them (this is different from trying to beat them).
If you treat a CPU as such, and take that with you in to local player with actual players it could only help you. People who cry out that any sort of gameplay with a CPU just "creates bad habits" are parroting, it only does that if you let it.
I'm actually really curious to see Amiibos in action.