I don't see it as necessarily a bad thing BTW. Obviously they make more money from doing this, but they potentially take risk adverse people who were buying gold into their ecosystem. It does have the unnecessary side effect of getting some people who never bought gold to begin with (because of risk) to actually buy it because it's legal. No numbers because there's no hard data and citing something would be stupid on my part. Things that were illegal before with risks that are suddenly made legal doesn't mean loads of people are suddenly going to buy gold.
It absolutely will cut into botting and people selling for RL money. How much? I have absolute zero clue. What it will do is push some bots and black market sales from happening as it won't be cost effective as it once was. Some will absolutely adapt and be forced to offer lower prices, or will adapt like they always do and find ways to circumvent ban waves or account closures like they always do.
For me I still won't buy gold with the token, and never have. It does offer me compensation for my time in WoTLK (I play retail and classic) when I decide I'm no longer having fun in WoTLK by purchasing subscription time. I fully expect GDKPs to be way less prevalent in Cata/MoP like they were originally if they decide to go forward with it post WoTLK, making gold pointless. 10/25 man having the same lockouts, having the same gear, 10 man being easier to organize and the game just being a bit more difficult (with every patch, even with solved content) points to gold not maintaining it's value like it does in currently WoTLK.
But again, that fixation on gold and purchasing power in WoTLK only really applies to players that participate in GDKPs to begin with. If you play classic the 'intended' way, you would just play in a guild and gold being absolutely useless.