While bad may be too strong a word, it's certainly a little odd sometimes, mainly because it is different to what, I'm sure, many of us are used to.
When you play a videogame, for instance, you have mana points. With D&D, it's like you're using a revolver. You load bullets into the cylinder, and while you can "swap down" bullets (say, put a .38 special round in a .357 magnum), you can't "swap up" bullets (Say, put a .357 magnum round in a .38 caliber revolver).
The "recharge after 8 hours of rest and one day has passed" mechanic encourages the 15 minute adventuring day, in my opinion.