DM has a great point on the 10 second rule. Its mostly about the officials and players getting set. I don't want a middle aged umpire blocking the mike LB or having to avoid him (mentally or physically).
And its something that I think TSUN and others get away with too often. There are MANY times where TSUN should be called for illegal procedure when they are not set before the snap. You can review their games and see a WR moving off of the line or getting onto the line at the snap. But, its a situation where the officials are not going to call that every play. Its like the Seahawks in the NFL. They have taken the stance of 'we are going to make the officials call PI every play. Every play they don't call it, we get the advantage'. Not that I think TSUN necessarily gets an advantage out of this. Its just about consistency.