Or he considers the risk acceptable, after all, he's got 5 others. And I'm not sure if you're supposed to be able to get your piece of soul back from a horcrux, but maybe when the horcrux is destroyed it just flutters in the air waiting to be shoved back into either its regular body or a new horcrux, and only dissipated if you don't do that. If that was the case, he might want Harry killed only when he is present himself, so he can recapture the bit of soul.