Too often I go to forums with spam being the main contributing factor - everything from hosting companies, forum invits and "join my group" type places. Facebook is a haven for spam and games etc too - hate it, and try to not contribute to it.
Broadcasting a business name over and over can be hurtful to the reputation as Romes pointed out. I honestly can't see how posting a handful of times in a row with the same message gets anyone's attention to actually sign up with them ya know?
I try to be careful even posting my own company advertisements at a minimum of 1 week (if it's a busy site and within the rules) but more often I'm posting maybe once per month. If I wanted more exposure in a particular forum, that's what a banner advertisement is for
I also have no idea why a forum that is loaded down with spam and almost NO content, why a user would bother to register and then spam themselves the same stuff. I know some is automated robots, but some are real people - it makes no sense.