Honestly, this service would probably be a good fit for someone like me. I own a console but rarely, if ever, buy a game. As a result I rarely play anything. I don't want to spend the time and effort to research games and figure out if something is worth my $30 or $60 or is just a steaming pile of crap.
I could see myself paying $5 a month to play a back catalog of games I would otherwise have not found. Discovery is a key aspect, but you know what, if I spent a week or two playing a less than stellar game I wouldn't really care all that much because it cost me what, $5 or so? No biggie. I don't care about the latest AAA shooter or playing the same Madden year after year with different uniforms. Just give me some decent entertainment at a decent price and I'll pay. Hell, I pay $50/month to run on a treadmill and sweat. I'll pay $5/month to sit on my ass and be entertained.