Since I haven't been PNG'd yet for telling Hitech how he should run his business, I thought I'd keep on trying.

In 1999, I believe most of the online air combat sims were hourly rate. $6/hr, $4/hr, $2/hr. In a lot of ways that was a horrible model as it set up perverse incentives for someone to not want to play. That money meter was going tick tick tick racking up dollars as you tried to have fun.
When HTC started Aces High with a flat $30/month charge, I thought that was brilliant! You could play until your eyes bled and didn't have to worry about a surprise big bill. They saw the future and the other games quickly followed or went out of business. When they went to $15/month I thought they had gone loco! But I guess they found a sweet spot. That was the perfect business move at that time. 17 years ago.
And for the hardcore players here still, who play a ton of hours, that is an awesome deal and they should keep it.
However, I think they are unnecessarily losing too many marginal, casual players that play intermittently, or go for periods of not playing due to real life. While HTC has created a good interface for creating and deleting accounts, I think you can even call and "suspend" our account, but there is still a bit of an annoyance there. And I think there is some friction caused by someone hesitant to open an account because if they don't play often, they feel they are not getting their full moneys worth. So finally they decide to close the account. Once closed, it's real easy to lose a customer for good.
What I'd propose is a middle ground between hour by hour and monthly bill. Keep the normal $15/month model for those that want that. But add the ability to by blocks of minutes. Like maybe 2000 minutes for $10. Maybe a logon message reminding them when they are below 30 min to buy more or sign up for flat rate monthly. The minutes don't expire. You use them when you have the time. Across one or many months. Makes a great gift card thing too. Granny gives her grandson a 2000 minute Aces High gift card.
That way, I may use them all in a month of heavy gaming, or if I get busy, come back in a couple of weeks and continue to use them without feeling like I'm wasting value when I can't play.
It's psychologically different for the user than a money meter. It's a sunk cost. I've already bought the minutes in a block. I might as well use them. From HTC's point of view, you lower the friction and maintain contact with a customer longer. Also, they at least pay for the block of minutes up front, so you get your money right away. That way you give them less of a excuse to close an account. That means you give them less of a reason to uninstall the game. You keep marginal customers that don't think they fly consistently enough to justify a monthly charge, but you are still able to monetize them and encouraging them to keep an account open.
And once again, I am not suggesting getting rid of the $15/month plan. Just adding an alternative block-o-minutes model for those players who's life that fits better.
$0.02.