A lot of players get very disengaged by their character going 'I Fire Bolt it.' 'I Fire Bolt it.' 'I Fire Bolt it.' repeatedly because they have to budget their abilities.
As I DM I make it my responsibility to vary the encounters so that the characters can make use of other abilities. Due to time constraints, the party generally has fewer but more difficult encounters.
If things go south and the players end up blowing a lot of their abilities in the first one or two encounters, they feel like you're picking on them as you drag their characters through the rest of the gauntlet.
My group will take an early rest and say, "well this day is shot" if that happens.
I also think that 6-8 encounters does violence to the narrative of action-adventure fiction (since it's a D&D-specific trope that doesn't have genre or metafictional justification) that can only be justified as a gameplay/story tradeoff but that's a separate discussion altogether. Just speaking from a gameplay perspective, it disengages certain kinds of players and I'm getting rather tired of boards like these treating such players as powergamers or n00bs.
I think you paying too much attention to that "6-8" value. I do whatever makes sense given the context of the story at that moment. It doesn't always make sense to have so many encounters. Do what makes sense.