I'd say that it depends on what kind of encounters you have and your style as a DM. If you have one encounter per day and no time limit, then nothing is stopping the PCs from taking extended rests. If you have the old school, Diablo- style dungeons where the monsters don't keep track of where you are, you can take extended rests even within a dungeon.
Here is the big part for the DM to counteract multiple extended rests: If the monsters are reactive, six hours are a looong time for them to mount a counterattack. If there are monsters enough in one place for six encounters, the only viable way to take them out is by speed. You strike them one group at a time employing surprise and denying them the opportunity to mount a coordinated defense. A D&D combat is short, a short rest is also something you can do (if you stretch it) before the enemy can localize you and organize themselves.
If you attack one of the six hobgoblin groups (conveniently big enough to be challenging for you), defeat it and then go back for a six hour rest, they have time to pull all the other five groups together and hunt you down. Assuming you don't have Aragorn in your group, they are bound to find your tracks and attack your camp. Then you are outmatched and will have to run or die, or run and die if they are faster.
If you use something like Rope Trick or if you have Aragorn in your group, they can pull back or go into general alert. The next time you attack you have daily powers and an AP, but the enemies can react very quick to your next attack. This makes the attack harder to do after a long rest than it would be without the long rest.
So if you are doing the basic go-into-dungeon-vs-intelligent-opponents routine, you may not afford an extended rest.
EDIT: The time needed to organize a defense is something that fantasy litterature is often underestimating. I remember a George R R Martin book where an entire army at camp got organized into formations, ready to take an attack in 20 minutes. It would take at least 20 minutes just to get everyone to understand that the enemy was approaching.