Given that GA tickets are the best tickets and are at the cheapest price, I don't see why someone would want to purchase any other ticket. In terms of just getting any ticket to a show in general, its not an issue for stadium shows depending on the market. I did have a friend in France use my special code back in 2005 because otherwise he would not be able to get tickets in the general sale for Paris. I did not have good luck with pre-sale code in 2005 and thought he could get better use out of it. That was 2005 though, the site was brand new, and demand vs supply of tickets was a huge mismatch.
I've never actually purchased merchandise from U2.com. I just have the membership for the pre-sale tickets, the annual gift, and the stuff that is available on the website for free.