BardStephenFox said:
I once ran a portion of an adventure where the party was being assaulted every 2-4 hours. They did fine until they started running out of spells. But, when your Arcance spellcasters are not getting enough sleep to get spells, your clerics can't pray without being harassed, and the whole group is starting to suffer fatigue, it starts to be tough.
Attrition can be great if the players and DM have all planned for it, and if both sides have the same idea of how many encounters can be expected in a typical day. It can really really suck the rest of the time. I speak from experience here, of course; that second possibility ends up being far more common than the first, and it isn't alway the exciting sort of thing you describe.
If the players were only expecting two encounters per day and instead had four, the spellcasters will be a liability in those later two, with no resources and a lot of vulnerabilities. So, the DM either has to fudge the scenario, shifting the last encounters to after a rest period, or he has to accept the very real possibility of a TPK (which isn't fun for anyone, no matter how sadistic you are).
On the other hand, if the players were pacing themselves for four encounters and you only threw one Big Bad at them, the spellcasters won't feel like they have contributed very much. They'll have held back a good number of big spells solely on the possibility they'll need some for later.
It's not just a question of "did you screw over the party?". It's more about the relative balance of casters versus tanks. In a 1-fight-per-day type of adventuring (see also Scry-Buff-Teleport), casters dominate, but in a prolonged dungeon crawl with no safe rest, the tanks shine.
***HOUSE RULE ALERT***
The whole concept of attrition was one of the key behind my friends and I developing our own homebrew system loosely based on D&D. The spells were the same, but our three magic-using classes are all at least partially attritionless.
Mutant: (based on 4CTF's Hero class) gets spell-like abilities that can usually be used at will, plus a bunch of always-on abilities.
Channeler: Every spell cast deals damage, so if you try casting a bunch of high-level stuff at once, you'll probably die. Between encounters, you can heal yourself back up, so there's no limit on fights per day
Wizard: Sort of a cross between the 3E Cleric and Wizard, with a Focus, specialization, material components, and some limited spell-swapping ability. Still uses spell slots, but every level or two can pick a low-level spell from within his specialty that he can use almost at will, without spending slots, IF he still has his Focus in hand. Even a first-level Wizard can pick one cantrip, like Ray of Frost, to be used at will once the memorized stuff runs out. A 20th-level Wizard might have one 3rd-level spell, two or three 2nds, etc. that he can use at will, all within his specialty. Someone who can cast a Cure spell at will, or offensive abilities like Fireball, will never really be "useless".
Anyway, the point is, we went to a system where the attrition simply never happened, so as DM the exact timing of encounters stopped being so critical. There are still good reasons to want 8 hours of safe sleep, of course, but it's no longer so ratbastardly to attack the players in the middle of the night.