Gah, I keep posting and being wrong My apologies to all.
What about the argument that it isn't necessarily the DM who chooses how many encounters per day the party has? When the group is out of stuff and wants to stop and rest, they'll stop and rest. Does the DMG say anything about letting parties stop and rest?
Not that it matters what the DMG says, since we're all intelligent people and know how to run a good game, whatever the book may or may not tell us to do
Its actually not completely clear how many encounters a party should be able to go through. I mean, I could see plausible arguments for anywhere from 4-10, really. Its going to depend on the relative level of the combats, and also on how much depletion of party resources each encounter of a given challenge rating is supposed to eat up, but the DMG is remarkably short on guidance here.
Just letting the players rest whenever is problematic in two ways:
(1) it shouldn't be all or even most of the time, but occasionally attacking them while they're trying to rest is cool.
(2) if *players* don't have a good feel for when they can't take another challenge, they can kill themselves pretty easily if the DM *also* doesn't know if they can't take another challenge.
Finally, the level of the next challenge may be crucial. If your next fight is the BBEG, but you don't know that yet, and you figure 'we can handle one more normal encounter', boy are you in for a *surprise*.