This is definitely something to consider, especially on the tournament scene, and if you check out my battle reports I have a few of these match ups to showcase.
My experience is that against dark angels, one unit of dark reapers can take out a small squad per turn, which really hurts them. Then just focus fire lasers and spiders into a second squad, and assault a third with your Wraithknight. If you have mobile d-scythes, go straight for the warlord. If you have perfect timing, even better, but you don't need it.
Actually against the ravenwing bikers, if you can soften them a bit, just assault those suckers -- in combat they are just 1 wound marines again.
For a batrep you can check out my game 3 from TSHFT in August -- I'll find the link for you later.
Against flying daemons, try and take out all their obsec while keeping yours alive, and win on mission points. It's practically impossible to kill them without freakish luck unless they come to ground.
Tau are looking like the next cover save army, but psychic shriek, perfect timing, and any assault presence will just ruin them if you have a few tough elements to throw right in their face. Batrep 5 from TSHFT is a good example of this one.
Not sure what your list is, but I'll assume it is pretty mobile and has lots of shooting
having a Wraithknight is a big help, failing that a hemlock is also good.