Well, it all depends, partly on what you mean by ads. I allow my members to post ads selling car parts they have or whatever. For 2 sites, which I actively sell advertising on, I prohibit the posting of ads by members who are also businesses. (I wouldnt sell ads if anyone could promote their business) Here, People are welcome to advertise their services (so long as it isnt spammy stuff)
SO it depends on their needs.