It does make Dev's more appealing- but I've been put off Dev's for a long time now.
4th Ed is mobility and firepower, hence why Mech Tau and Eldar are in ascendance. And yes Obliterators and Predators can move and shoot (IW's) with decent weaponry so fall into the M&F too.
Dev's are static- on a 3rd Ed board with no terrain, they are nasty. Deploy, sit, choose target- roll dice.
On a 4th Ed board with proper terrain (25%+), the mobile firepower opponent is rarely dumb enough to sit in front of them- unless he intends to kill a full squad in his turn (by concentrating his firepower).
Being able to split into 2 5xman squads with 2 heavies- does alleviate this a bit. You can cover two areas rather than one, etc.
But also, the mobile firepower opponent will just move to see just one of these squads and only have to kill 5.