RAH, I think that if someone is going to poach a deer, it won't make any difference to them if using a crossbow is legal or not.
I'm 110% in your camp on practicing to become proficient or don't hunt with that weapon. All sorts of things can happen to make a bullet or arrow wander from it's intended course even if you are an expert marksman. It's even worse when you compound that with someone shooting that can't shoot good groups.
I see many gun hunters "practicing" for deer season by shooting from a bench, using a rifle rest or sandbags. To me, that's not practicing, that's finding out whether the gun is accurate or not. Practicing to me means shooting using the same positions (and distance) you'd use when hunting.