I think this, as well. It makes sense, considering how problematic Dungeons mid-expansion have become for Raiding progression, over the years. By the looks of it, Scenarios and the Heroic versions will fill the gap that mid-expansion Dungeons will leave, which will allow for more diverse gameplay than the typical 5-man content since it can potentially range from solo play to up to 40-man play in group size.
I find that aspect of Scenarios to be quite attractive and if they manage to pull off more stuff like the Green Fire Scenario for Warlocks, I think I will enjoy it quite a lot.
Edit: Also, since they said that there will be larger raids, I think the non-dungeon decision is a direct result of that. Can't say I have much reason to complain, then. =P