Well, depending on how overt the advertising is, I'd imagine the reputation of that user would fall quite quickly. It may still be deemed that it would be a worthwhile investment, though I'd assume a quick negative reaction through negative reputation would perhaps lead to questioning if the move is hurting or helping their goal of advertising. I'd also imagine users that suddenly start spamming a lot would end up getting banned, possibly before the reach desired was achieved, which could create disincentive for going this route (very unreliable ROI).