Yes, I register/transfer my geldings when possible.
As for why, value for one, and yes there are some fantastic grades out there, however, two horses completely equal, one with papers one without, which do you think will sell faster? So you have registration papers, but don't transfer, isn't that enough? Maybe, but I can say from experience that having a horse who's paperwork isn't in order, can be a nightmare, so personally I prefer not to buy a registered horse whose papers are not in the name of the person I'm buying from.
I sold a horse last year, a registered 10 year old gelding, only to see a pic of him a few months back with a new owner wanting to know if anyone knew him. Apparently since I'd had him his papers had been lost and he'd lost 4 years (she was told he was 7).
I like registered horses because I like proof of age, breed, and because in my opinion a registered horse is a little more valuable if I ever deside to resale.
That doesn't mean I won't own a grade, I just know when I buy one I have to take what I'm told with a grain of salt.