Don't you just hate feeling you've been suckered? Would you want to inflict that feeling upon your customers?
I received an interesting report from a coworker recently. He was searching for AVG on download.com (now download.cnet.com):
He selected the version he wanted and was taken here:
Expecting a big green download button, he clicked the one at top right (not realizing it was actually an advertisement) and was surprised at where he ended up:
When he realized he'd been redirected to PCTools and a download page for Anti Spyware software he wasn't interested in, he was understandably miffed. Had he not been paying closer attention, he might even have started the download... which would have made him even more miffed!
I don't suppose we'll ever see the end of such misleading advertisements. Unfortunately, they work too well. But if you're interesting in improving your user experience — if you don't want to alienate your visitors by making them feel you've suckered them — you might want to think twice about allowing such ads on your site.