The first advice the DM guide give for building encounters is that encounters should be fun for players, the second one is that encounters should not be a burden to run by the DM.If balanced is borked if you don't follow the guideline, it pretty much as the force of a rule.
So you can claim this second important advice to not running 6-8 encounters a day, if you consider this as a burden. And you also claim that having 6-8 encounters may be unrelated to players fun.