I disagree with the conclusion. The reason that I use the 6-8 encounters is because there is a large amount of empirical evidence across a lot of tables that shows that this is the point the at-will, short-rest-recharge and long-rest-recharge classes balance against each other as well as common attrition tactics take a toll.
Fewer, harder encounters favor some classes more and disfavor other. Number one common element in "5e is easy mode" threads is that they don't do 6-8 encounters.
BTW, I'm not defending 6-8 encounters - I really enjoy 5e and it's by far my biggest complain about the system. I'm just saying that the reason the 6-8 gets talked about is because not only do they suggest it, but it holds up. Like as if they balanced against it.
I agree this section is worded horribly and lends itself to confusion. [MENTION=6783882]Nevvur[/MENTION] correctly quoted the DMG, but in your response you are already dropping the key words of CAN along with MEDIUM TO HARD. These are all very important factors in the guideline.
When you look at the Adventure Day XP guidelines, you quickly see that what is missing from the 6-8 encounter assumption is that 3 Deadlies also give you the full adventuring day XP and equal the amount before a typical short rest. So you CAN get to the recommended maximum in 3 Deadly fights (with a short rest between each).
Now, I know there is an argument that favors LR classes. But in actually play it doesn't. Sure, a Barbarian can theoretically rage in every encounter if you are going 3 Deadlies, but that assumes first that the Barbarian knows there is going to be exactly 3 encounters and that there would be a need or desire for the Barbarian to rage in each of 8 medium encounters.
As for LR spellcasters, the fewer number of encounters per day is balanced by the fact that the number of combat rounds per day stays roughly the same. A deadly fight typically takes 5 rounds, a hard 3 rounds and a medium 2 rounds. That means that the party can handle about 5-6 rounds per short rest and about 15-18 rounds per day. Running TOM, there is no reason that a PC turn, even at higher levels, should take more than 90 seconds to resolve with the DM taking a max of 3 minutes, meaning that a full 3-8 encounter, 18 round day should have less than 90 minutes of combat table time.
To sum up, here are 4 points to keep in mind for the Adventure Day
1. The guideline in the DMG is designed to warn DMs about the MAXIMUM number of encounters a party can handle before running out of resources.
2. The guideline is for 3 five round Deadly fights, 6 3 round Hard fights, or 8 2 round Medium fights split into 3 sets of six round combats per short rest and 15-18 rounds of combat per long rest
3. Balance between classes only requires that PCs believe that they could face the maximum daily xp. If they are going Nova too often, you may be too predictable in your combat encounter structure.
4. If as a DM you haven't tried to run 5e using the Adventure Day XP guidelines from the DMG, try it!
Finally, I routinely throw my PCs into situation where there is far more XP between them and their goal then they could handle by taking a kill them all approach. Only by trying to avoid combat can they hope to reach their objective. This really focuses them on using resources wisely throughout the day.