Where should we draw the line with genres? I think it goes a little too far and its pretty apparent with the actual need for a separate genre tag for "Melodic Metalcore".
Let's face it, the only significant difference between Melodic Death Metal (MDM) and Melodic Metalcore is the addition of breakdowns, sometimes choppy Metalcore riffing, and at times Hardcore like vocals. Sure a number of bands use clean vocals in the chorus of their songs but MDM bands have been doing this since the late 90s. Go listen to the In Flames album Colony or Soilwork's A Predator's Portrait. Though obviously as simplified as most Melodeath is, the Hardcore influenced bands seem even more so streamlined.
But still, does it warrant its own sub genre? We have bands like Children of Bodom, Brimstone, and Norther who play MDM heavily influenced by Power Metal but we don't give them some new name. Or the Melodeath bands influenced by Thrash like The Crown, Darkane, and Thine Eyes Bleed. I don't think Melodic Metalcore bands are distinct enough to be labeled outside Melodeath.
Bands like Shadows Fall, As I Lay Dying and Killswitch Engage use the twin leads popularized by Iron Maiden and which influenced bands like In Flames, Dark Tranquillity and At the Gates. The drumming is practically the same, though I find it funny that the Hardcore tinged Melodeath bands tend to be more likely to use blastbeats.
So seriously, for the sake of simplifying the craziness that has become over genrefication of Heavy Metal, let's just keep all the bands under the Melodic Death Metal umbrella. Yes, let's keep bands like Nightrage, Trivium, At the Gates, All That Remains, Dimension Zero, and Unearth in the safe grouping. Its ok I swear, you won't die seeing Still Remains lumped in with Gates of Ishtar.