I don't think all that many people will drop their regular professions to pick up herbalism and mining just in order to get a boost in XP whilst leveling. Unless they planned on keeping both, they'd just loose all the time they "saved" leveling their regular professions back up to max.

Not likely as many people will need to level Blacksmithing, Jewel crafting and Engineering. I hope you are right though as I will be leveling all of those on many toons. My DK Miner/SKinner loves the idea of XP for gathering :) Then again after looking at changes to the Blood and Unholy trees he has a good shot at being 3rd on my depth chart :)

Plus there are only so many people that a zone/world can handle that are going gathering crazy can sustain, ask anyone who took part in the saronite mining rush in Sholazar.

In the game currently, you get experience while leveling up skinning. If people aren't currently rushing to pick skinning as a profession now, there's no reason to think they'll be rushing to pick up mining and herbalism in Cata.