I think in general the env_explosion entity is pretty borked in most Source games, the only instance I can think of it being used is the bomb sites in CS:S.
From what I understand, the rolling explosions in GE:S are a replacement for the HL2 rocket/grenade explosions, whilst the env_explosion entity actually produces a different kind of explosion entirely... Though I'd love to see a GE:S explosion entity that can be used in maps, that would be great, and pretty much essential in the future if singleplayer goes ahead.