I highly recommend signing up. You get a $25 credit as soon as you sign up so that pays for itself. The real bonus is not the credits earned from spend($25 for every $250) but the emails you get for the specials.
I signed up at Mortons and I routinely receive emails regarding the specials that are only for "preferred guests". The recent ones that come to mind were the 34.99 Halloween deal, various 49.99 deals and of course, the 34.99 for a 2lb Lobster deal.