mim they do when fighting Grey Knights. .. At least in 4th ed they, havn't kept up with gk since then.
Arguing with a chemist is a good way to win a Darwin award!
LOLWUT? Eldar don't summon Demons. Even Dark Eldar don't do that. Eldar might field an Avatar, which is roughly the same size as a Carnifex. But...
NB: There are both 40k and apoc formations, the apoc formations are not for use in games of standard 40k