AT 1500pt game 3 wounds is quite effective, esspicaly if you have a little bit of shooting in the rest of the army to finish it off if needed.
Sorry, what I mean is that there is multiple points of failure...
1) You need to get the deathmarks on from reserve. (66.66% chance, 0% on the first turn)
2) You need to deepstrike them within 12" of the target and not scatter outside of this range. (otherwise you're only inflicting 1.6 wounds on a TMC)
3) The Deathmarks are left within 12" of the enemy unit they failed to kill, and the TMC should be able to wipe them in CC.
4) On the second and subsequent turns, even if the Deathmarks are at full strength, they inflict 2.2 wounds (or 1.1 if outside of rapid fire range).
For almost the same amount of points you can get 3 destroyers who will cause 1.8 wounds on a TMC, each turn, every turn, from beyond 24". The destroyers are more mobile, better against MEQ, GEQ, light vehicles etc, not subject to interceptor fire and can JSJ.
The only time the Deathmarks are better is the turn they deep strike, as long as they are within 12", any other time the destroyers are a better choice assuming you have free slots for them