MMO Subscription models?

So with the news of Wildstar's payment model option and ESO's, I've been asking myself a question I've asked myself since I first wondered this; Why do daily subscriptions not work?

Like, why is the case that I can't purchase 30 literal "days" and only have my available play time deducted when I log in for a given "day". Call it whatever and say either a day based on local time or a centralized time, or a 24-hour period of my initial logon.

I buy 30 days. Log-on at 3pm Tuesday, consume a day (29 left). Log-off at 5pm Tuesday (29 left). Log on at 2pm Wed. still same "day" (24-hour window, 29 left) and if it slips pass 3pm, that's another day (28 left). Log-off at 5pm Wed. (28 left).

Next log-on is Saturday at 12PM (27 left). I play all weekend and Log off Sunday at 5PM (26 left). Next Log on is Thursday at 8PM (25 left).... you get the idea. Why does this idea not work... or what I really wanna know is why has it not been implemented? (as far as I know).

I understand that a chunk of it may be due to monetary gains. You make less if it's more versatile. If I don't play a day, or for more than an I feel Like I've wasted 1/30th of my sub fee. I know in other areas these kind of sub ideas are hard to implement or manage, and granted, I feel that used to be the way with MMO's too. But watching the growth, I wonder if this is not feasible now?

I mean many will still do a cash shop either way, and I'd be first to say I'm far more open to a sub if I don't feel like I've wasted my money if I'm not a die hard with tonnes of free time. I like Wildstar taking an initiative but it's an old hound in new furs. Both are still a cash grab, and while gold can still buy a sub, the round about economy will suffer, I feel. So why can't the daily log work?

I'd rather pay for thirty days and know I've played 30 hours (say an hour a day) at my leisure over two months, than know that I payed for a 30-day period and played only 15 hours. Or worse, I played 47 because I feel I "have" to (and to be honest, that's why some people drive so hard in MMO's, I feel. Smaller reason to some though, I'm sure...).

I've never heard it discussed as a possibility, I've never heard major reasons not to do it this way, so why hasn't it been overtly explored? Are there any among the forums to discuss this and/or shed some insight on this thought?