I would have to say that hitting off a tee definitely helps. One area that I have noticed this helps if done correctly is improving use of the lower half. Of course you need to be smart when you do this, such as changing the height and position of the tee. If a hitter is struggling with a mechanical issue they can use BP to fix this. The problem with BP is that young players use this to show off raw power, rather than working on going gap to gap and hitting line drives. In the end, player development boils down to being all about communication, with players and coaches needing a two way street where both can comfortably have a dialogue about the issues they are having. If neither can do this, they are probably not using their mental facilities and therefore should not be in the professional ranks.