And just what makes you think that the PCs are all going to sleep at the same time? Rather than in watches?
I would definitely expect the PCs to do that, but how does that change my argument? If the PCs are the only thing blasting the undead back, how is only one character going to hold them back? If my PCs were exploiting the "full heal", then I would have the undead launch a larger assault in order to knock them off balance. There are numerous ways around the "problem" of having people heal once a day over an 8 hour period. Remember, it has to be continuous. If it's broken in any way they have to make up all the lost time.
I think the problem here is that it fails one of the fundamental rules of good design.
Which is what? It seems to me that a lot of people like one way and a lot of people like the other way. If it failed a fundamental rule of good design, you would think that everyone would hate it because it's
broken.
To do this you'd better have 4e level balance mechanics. Because a new DM is going to have problems threading the needle. Or he's going to have to keep coming up with excuses to launch yet another attack after four hours. Rules which make the extended rests take an extended amount of time don't force a new DM to navigate between Scylla and Chrybdis here and therefore don't encourage the DM to DM badly.
DMs have been threading the needle for over 20 years before 4e came out, so how is this any different? I've always allowed for full healing when they rest for a full day. If I don't want them resting, I keep them on their toes. It's really not the difficult.
It wasn't the mechanics. It was the design of adventures. Dungeon-stomps are a limited type of roleplaying. Yes, you can have an excellent game that involves dungeons. But you can have an excellent campaign without an actual dungeon to be seen. If it's either-or then I'd rather have no dungeons to just dungeons (I actually prefer a mix).
Ok, then it isn't a problem of D&D at all. It's a problem of the pre-made adventure designers. I've never used a pre-made adventure. Therefore, I've had a good mix of outside and inside adventures.
Remember that all of D&D, every bit, is simply a base ruleset. Everything else rests on the DM. If you don't like full healing, then don't allow it, make up your own rules. I don't particularly like magic items. Therefore in 3.5 and 4e, I lower the power of high level enemies because of the deficiencies of PCs with no magic items.