In combat your most powerful abilities work once a day, and the players have (some) control over how many fights they get into each day. The emergent strategy, barring DM intervention, is to blow through every available resource, every single fight. That's a fault of the system more than it's the fault of a player who takes advantage of it.
But's that's only if you look at the combat in a complete vacuum. I mean, there's a whole campaign world out there, and if there's really
nothing going to happen if they just take a day per encounter, well, isn't that just fine? So they're in a scenerio where they've ample time, enemies aren't communicating with each other nor preparing defenses (so there's no reason to hurry), they don't have a reputation for quickness to uphold, and fights are dangerous - this doesn't sound like a "quirk of the system", it sounds
reasonable in-game to be careful and rest well between those kind of fights.
The way I see it, the problem doesn't exist from a fluff or storytelling point of view. If there were a problem, it would manifest itself somehow.
In-game.
The problem is a purely mechanical one, and the problem isn't that they're resting - resting a lot is perfectly reasonable! The problem is that the "default" adventuring day
balance assumes that they'll do several fights a day, an assumption which is usually valid but not if time is completely irrelevant. The solution? Don't assume that there'll be several fights a day; simply make fights you expect them to rest between harder.
But really, it's pretty rare IME for there to be absolutely
no reason to try and hurry. Usually, there's
lots of reasons. If they're being payed to clear something up, and sleep a month, their contractor may well assume they're dead and hire someone else. Hey, we've got a plot hook! Whoops, he's got the reward now, and it's unique - he
claimed to have solved the problem, but he must have stalked you guys. You'll have to claim your reward from him, but the only clues on his whereabouts I have are these...
Or damn, it's winter now, and hell has frozen over; try again next spring...
Or the BBEG has succeeded in raising his first undead legion and overrun the lowlands; your only chance now is to sneak past his guards and destroy the crystal of plothook animating his legion (but don't take too long, he's not sitting still...).
In fact, even if there's no
obvious reason to hurry, if you've encountered this kind of stuff regularly you'll still avoid needlessly wasting time, since you have
no idea what else might be going on.