There are two issues with the 15-minute adventuring day:
- People getting all cute, knowing that they are players in an adventure, or that the DM doesn't want to have too many wandering monsters, or other such--and resting all the time for maximum effectiveness.
- The feel of being a party of adventurers that is doing some dangerous thing, where the proper moment to rest is a critical decision at times, and you'd like to be able to push on when necessary.
The first part is mainly either a "social contract" issue or miscommunication between DM and players on expectation or perhaps baggage from previous adventures or even DMs. (Players in campaigns with "Killer DMs" can quite naturally turn into "turtles," for example.) The way to fix this is a mixture of talking about it to set expectations and some reasonable procedures by the DM to make resting all the time not a good idea--e.g. time pressures, wandering monsters, etc.
The second part is affected by those same kind of DM tricks, but if the mechanics are too blatant at leaving all of this up to the DM, it can start to get a bit hollow in certain playstyles. Maybe the group
has talked about it, and has decided they are playing "big heroes," and they don't want to have to fool with wandering monsters or time pressures or the like. So the "social contract" says that resting after the first skirmish with the kobold scouts isn't going to happen because "heroes" would press on in that circumstance. And that can more or less work for some people, some times.
Everyone has a limit, though. Eventually you end up with a situation that didn't go totally awful, but did go much worse than expected. So now you are forced to talk about it in social contract terms, in the middle of the game, because that is what you base the decision on. Or, every player has to guess. And you get comments like, "There is no good reason why we shouldn't rest here, in game, but I guess we'll push on."
I have found, at least with my last three groups, that a little help from the mechanics can go a long way, here. I don't need a reason strong enough to curb a table full of powergaming, rules lawyering, DM mind-reading players trying to determine the optimum way to dominate the adventure while minimizing all possible risks. I wouldn't play with such a group. We do appreciate a "reason" good enough so that the
players can tell themselves, "We press on because otherwise X." That way, even when there isn't time pressure or much risk of wandering monsters or any other of those things, we are
encouraged to stay in the genre that we want to play.