They'll never do this, they'll never go with a per-day subscription reimbursemant system. It would be too hard and lose them too much money.
-for the same reason magazine/newspaper subscriptions don't do this. it's micromanaging and micromanaging costs more $ in the end. BUT, what if they had a weekly sub option? It might be marked up because of convenience (smaller lots of time = higher price per lot, larger lots of time = lower price per lot, I.E. those 3- and 6- month packages most MMO's offer lately), but would leave some doors open for people who only have 1 week out of the month to play.
1 week = 7 days, $0.50 per day = 3.50 raw, mark up to $4.99 a week
I thought WoW in some Asian areas was hourly or daily rates instead of monthly subs? Also, you can buy a single copy of a newspaper, not like you have to sub. Also, I only get the Sunday edition of my local paper, so they do have options.
Now, I don't see them going to system like the OP described because it would cost them way too much money. The OP is making the same mistake that the B2P people make. Ignoring the fact that this would apply to everyone.
So even someone like myself that will try to play everyday, some days I wouldn't have time to play at all, or would say meh, I only have half an hour to play, might as well save the money. So they lose money from the regular subbers, and I doubt seriously if they would make it up in people who otherwise wouldn't play. Not to mention the additional expense in tracking it as you pointed out.