Assuming there is an element of exploration in a given adventure, where the PCs are uncertain as to what the day will bring and what they might face in the way of challenges, this is, IMO, perfectly acceptable. To assume a dungeon delve, the PCs are going into a uncertain terriroty with a limited amount of information in order to explore it, knowing full well they are likley to come across dangerous obstacles. being prudent just makes sense, given the uncertainty of the situation. Should the PCs happen to find a secure location to hole up and rest before they need to, it is time to make a decision: press on, hoping to be able to either find another such place or return to this one, or hole up and get back to full force before moving on even if they aren't down that many resources. The game is about such decisions, and this one in particular is the perfect kind of decision for an exploratory game.
Exploratory dungeon crawling, hex mapping and other forms of "old school" play have kind of gone by the wayside in favor or action-adventure film pacing, however, and I can see where the difference in tone and playstyle might suggest per encounter abilities would be beneficial. But that doesn't mean per-day resources are "bad" or poor design as some have suggested, just geared toward a different sort of game. The problem is that by eliminating or seriously downplaying per-day resources has the effect of invalidating a playstyle, where simple includinga per-encounter resource model in addition to the per day resource model (for example, having both a Vancian Wizard and a Warlock as viable character options) would have broadened the possible playstyles of the game instead of limiting them.