Improved XP also?
I think the problem here is that pushing NM dungeons is a pinnacle activity so the xp from mobs is irrelevant at the cap. However it makes it feel pointless to do anything >3 levels above you at less than max level as there is no real incentive other than bragging rights. At least with the paragon system in D3 (or the gem system in PoE) there is a reason to grind exp even at the level cap. With Glyph exp being linked to dungeon completion the two systems are disconnected.
I personally look forward to their being a shortcut to the run, if its a portal that opens then you have the choice as to whether to run or not so anyone who wants to keep that immersion can do so if they wish.