Rapid fire on the tac is desirable to me because many guns require normal trigger function. It is easier to use the right tac for rapid than keep switching rapid fire on/off the trigger. Secondly, the modded controller I had before (Evil) had rapid fire on the tac, so I'm used to that.
As for the X button moving to left tac, my reason is that my preferred control scheme is Bumper Jumper (LB is jump, X button is Armour Ability). It would be nice to have X on the left tac so that I could use Armour Abilities without taking my hand of the right stick.
I can't use the Preset to make my own as it doesn't let me map X to left tac or rf to right tac.
You are right about Reach and the different guns not firing smootly. I'm not too bothered about that though. The most important guns are the DMR and the pistol, both of which don't need a high fire rate. I only use rapid fire on them when up close because of the bloom.
At the moment I'm using the All in one preset with rapid fire on the right tac. Mode 1 gives smooth firing for the Assault rifle, so I pick that. Having X on hte left tac would be better though.