There's actually a few additional features you get by registering WinRAR (other than removing the nag), but they're not really useful for anyone, really. The people who buy WinRAR are mostly companies who really care to stay legal, and this is where they get the most money with. This is exactly why this helped making it one of the most popular piece of software ever: piracy is low since you don't need to hack it and this give them free publicity to get them in companies who care about legality where they make money.
Also for those reasons the pay what you want model is the most successful model for small developers because people who buy it genuinely care that you did a good job and not because they had to throw money to get access to the content, they might even give you $1000 because they have a lot of money and they think you deserve the $1000 and that might pay for 100 users who had no money to buy it in the first place.
So yeah, people might hate that feature of a program, the feature of actually buying it, but if used wisely, it's real good.