It is a successful tool for ad networks to provide because advertisers pay more for it, I've yet to be convinced that it actually increases conversions. If I keep seeing an ad over and over again, I just get annoyed.
I've seen the number, it converts far, far better than average ads. On the order of 10X, for the campaigns I've seen. That's partially because clickthrough rates overall keep dropping, but still, they're really effective.
Yes, it's annoying if you're in the 95% of customers who won't actually convert. But it only has to be effective about 1 out of 20 times to be a huge improvement (and that 1 might even be you, but you're mostly annoyed by the other 19 times.)
