From what I've seen, the only benefit to having both a crafting and a gathering profession on any character is that they'll have an easier time obtaining Blood of Sargeras reagents (which are BoP). That said, there have been a number of improvements during beta, making them not as hard to obtain for non-gatherers as they used to be - there is a shoulder enchant available and they also show up as World Quest rewards now. It will still be advantageous to have a gathering profession combined with a crafting profession, but I'm personally just counting on the leverage of being of a multiboxer to make up for the difference.

I do have the luxury that, since I like being able to go "Hey, let's give class x a spin and see how that goes!", I've made an effort to level one of each class to max level on all of my accounts, for both Alliance and Horde. So for me it's a matter of making a little matrix to ensure that I'll be able to take a mining team out for a spin, for example - though much like Wubsie, I have no intention of grinding gold by gathering.

That said, I am biased in that I highly value the use of the loot-a-rang, and since the non-engineering version will turn into a Draenor-only toy tomorrow, I've turned my entire army into engineers - which means my options are rather limited anyhow when it comes to combining professions.