If teams that didn't win their division or conference make the playoff, then it minimizes the importance of some of those in conference games.
And if you take only conference champs then the out of conference games mean nothing. Basically, you would encourage a team like Penn state to schedule Louisiana-monroe instead of Pitt in the early season.
The whole season should mean something. Out of conference schedules should matter and count. They don't in many of the revised proposals.
As for going to 8, one example of the regular season being devalued might be this year's Ohio State v. Michigan game. Would not have been nearly as interesting if it was clear prior to the game that both teams would make playoff. That is what would have happened with an 8 team playoff.