Advertisement on the forums, I couldn't care less.
Advertising in-game and it might be ruined in my honest opinion.
Of course, you can rent an advertising space and put up whatever you want in there, but I bet most servers' advertisement places won't be rent by players. Most servers will probably be advertising things that will possibly gain profit from advertising.
It wouldn't bother me that much if it was alongside with the menus or something outside the in-game fightings themselves, afterall the game is free and of course Nabi has to get money, but on the floor of the fights themselves doesn't seems as the best place to insert advertisement into in my opinion. It'll possibly screw some blood splatters, but the main factor, it'll be plain ugly in most cases to see a advertisement stamped into the shiny ground of the fights. I just hope it's not something enormous, nor something that messes up the reflections and blood splatters.
That's my opinion for now. And it'll probably only change after the in-game advertising comes into play.