If either one of them have a sub up when Darkrai switches in, he's not doing much back to them either. Espeon has Signal Beam to severely hurt it, maybe even KO with SR, and Xatu can U-Turn out harm-free, and both have access to HP Fighting, like you said.
On a side note, Darkrai can use Focus Blast, can't it? Why would it use HP Fighting?
Besides, with the new sleep mechanics, I just don't see Darkrai being used in the same tier as Espeon and Xatu. Yeah, Magic Mirror is a boon to both, but I still doubt they can compete in the same league as Darkrai, whatever tier that may be.