Depends on the game. As I said earlier, most micotransation games (at least all the ones I've played) are extremely play to win, and you do have to spend a ton of many to have any chance of competing against those who do
I understand your point though; if you don't have a job you're not always able to consistently come up with $15 every month, I used to be stuck in f2p games. But now that I have an income, I don't like playing them
I think someone should "perfect" a buying time system. Maybe like $15 is = 300 in game hours, but then you could still sub for $20 which has unlimited hours. But the hours never expire.
I think that'd be reasonably priced considering unless you're just no lifing the game it'd be difficult to reach that point in a month, it'd also allow people who can't afford $15 every month to actually plan accordingly.
Edit: I think this would be the best payment model for an mmo.
$7: 100 In Game Hours (Never Expire)
$15: 300 In Game Hours (Never Expire)
$20: Unlimited Hours (Monthly fee)