There's nothing wrong with electronic keyboards per se. Some of them can mimic the sound of an acoustic piano quite nicely. Or they can be used to provide nice bass lines. The problem with them is that they're capable of creating hundreds of different sounds and most of them just aren't appropriate to a session. If the player realizes that and plays with restraint, a keyboard can fit in just fine. If, as jargarmani said, he thinks he's Keith Emerson, he can ruin everyone's evening.