A level on your sight is definately not a beginner tool and can/should be a critical part of your shot sequence. Just look at which sights have the levels on them, the ultra low end beginner sights....nope, it is all of the high end sights, nearly every darned one of them.

The level ensures that you counteract any natural cant you put on the bow due to any number of outside influences (hand position, bulky clothes, shooting across the side of a hill, odd shooting position, etc....)

Here are a couple of really good examples of why this is important:
1) There are a number of 3D courses in my area that have shots across a pretty good slope. When you set up on the shot, unless you have a level, EVERYBODY cants their bow. This can lead to some pretty severe left or right shots.

2) One of the guys I 3D with went on a mountain goat hunt. He spent several tens of thousands for the hunt. Trained like a madman and was totally ready. He set up on a pretty good ram that was a little above and to the side of he and his guide. In the excitement of the moment, he did not pay attention to his level. Due to the extreme angle of the mountain they were on, his bow cant was bad enough that he missed the ram (shot went wide past the ram's chest). Once in a lifetime opportunity, missed by something as simple as not paying attention to a little bubble....