I totally agree with this. When I was questing i simply do each mission once but after finishing Alderaan i am already lvl 36.5, 4.5 levels over the planet.
I would say the ideal set up for mission is that after doing all the missions on a planet you can ALMOST get enough exp to fill that 4 level gap.
Again, i understand people have different play styles, but still, why put things there if they are meant to be skipped.
The XP is great, so great that if you were actually doing all heroic 4s by killing your way in, you'd have to end up skipping entire planets at some point to not out level them assuming you've no interest in doing quests that yield no xp.
So again, what's the point to do content that, if you did it all, would force you to skip a planet later? I guess you can do ONLY heroic + story + major quest, but what do you gain out of it? It's not really any faster than doing normal quests, and the gear reward is negligible (heroic 4 oranges are usually worse than plain blues of same level).