The mind game.
I'll use move X as he has Pokemon A out. Wait? What if he expects that and switches to Pokemon B to take that hit? Maybe move Y would be better? What if he stays in? Would switching to Pokemon C be the better move? What about using move z if he decides to do switch to Pokemon D, expecting Pokemon C. Maybe I should switch to Pokemon E. But what if I get paralyzed, or get hit critically? Then what would I do?
Honestly, despite all the Pokemon hax >.<, it is this sort of interaction between the players that I find most enjoyable, the constant battle of prediction, strategy and in some cases the completely unexpected surprises you kept hidden to unleash at the opportune moment to take the game.