I've never cared for the "HM Slave" concept. It gimps my team far more than simply having the HM moves spread out across my main team. Heck, the only really bad HM moves were Cut, Flash, Defog, Rock Smash, and Whirlpool, and only one of those is still an HM move in Gen VI (though Rock Smash still has its functionality, and I think Flash has a different function in Gen VI, since there are no super-dark caves). The rest of them were actually somewhat useful in battle. Especially Surf.
For me, I do plan out my mon's moveset quite specifically, and HMs really don't factor in. With a HM slave, sure I'm at 5/6, but I'd rather have that when wandering around than gimping the movesets for most of my team.
And flash still has use, as there were a few caves in Gen5 and maybe 6 that needed it.
Well, I usually like to at least have access to Surf and Fly at all times, since they help me get around and they're actually quite powerful. Quite frankly, in Gen VI, Cut is the only one that really gimps a team member.