In any organization you need to balance having a staff large enough to get the work done, with the fact that the larger the staff becomes the more cumbersome it becomes. More people means more time getting everyone on the same page. Too few people and you get overwhelmed, but too many can also be overwhelming. Often it is the case that a group of four or five people can get more done than a group of 12 or 15.
Also what is the role of this QB coach. Well two roles: (1) tell Tom what to do; (2) teach the new guys. I think we can all agree that number one is be handled fine and Tom knows what to do.
So that brings us to the mentoring the new guys. This would require considerably more skill at QB and at teaching than the role of coaching TB. No many people have the skill set to be an outstanding mentor to new QB, in fact it would probably require someone who was an outstanding NFL QB. However the Patriots already have someone who can and does mentor the new guys. None other than #12 himself. From everything I have heard, Tom more so then almost any others starting QB works with the his backups to help them improve and develop. It is a role he enjoys, is good at, and takes pride in.
Before you say, "but Tom shouldn't have to do that, he should be focusing on the game" let me explain that doing this HELPS TOM. As any teacher will tell you, the best way to truly learn how do something inside and out is to teach it to someone else. By teaching the new guys he improves himself.