I use max HP instead of special def on my assault vest set. Even with the -def nature, with that spread you can survive a +2 extremespeed from normal Arceus and deal 85%+ back with foul play. It's a nice emergency check, and you don't lose too much on the special side. Take this:
252 SpA Mega Mewtwo Y Ice Beam vs. 252 HP / 0 SpD Assault Vest Yveltal: 180-214 (39.4 - 46.9%)
252 SpA Mega Mewtwo Y Ice Beam vs. 0 HP / 252 SpD Assault Vest Yveltal: 144-170 (36.6 - 43.2%)
For comparison. Still a 3hko in both cases. Meanwhile, ekiller can only KO at +2 if he has life orb and adamant. If you're like me and rely on a burn to stop Arceus, but he's got a lum berry or you miss and he kills your burn poke, it's nice to have a backup check.