Another reason, is that, if the bore matches the ball, you should get better consistency between shots, which also aids accuracy, and improves efficiency.
A couple of companys use inserts, namely the Smart Parts Freak system, tho they probably stole the idea from someone else (but may not have done!)...but multi back barrel systems are also available, where you have a selection of backs in different sizes and one front section...the Evil Pipe, is a very good example.