To add to what Bryson said above, the way I look at it is that FGs and PATs are often the most important play in a game, so you want your best player on the field. Now, that best player may be your backup LT or your TE3 (long snapper) or whatever, but he should be out there. Because opponents do "block" kicks because of missed blocks. The reason that your best player may be a backup is because your blockers really only need pass blocking since, as far as I know still, strength isn't used in pass blocking calculations. (Someone please correct me and PM me if this has changed.)