I had friends in Groupon who used this technique. They claimed that forced sign ups didn't really result in a high enough bounce rate, compared to the number of sign ups they got.

Data driven does a poor job of capturing user annoyance. Someone will tolerate being screwed a few times to get a discount. But after a while, it builds momentum.

Though in many cases, startups don't care. They just want to sell off their numbers and exit.