Yes, this is universally true. Reason are three ways, starting from most important
1) Other slingers are stealing Hamorrage stacks. In real raid you actually use 3-4 stacks out of 10 for yourself and that is if you keep that in mind and are lucky.
2) You can't really depend on roll damage
3) Spec is really really bad againts low hp targets, also swapping is tricky. If you swap right before reapplying dots, its ok, but if you swap in second part of rotation, your Hemorraging Blast has to be replaced with basic attack, which is meh
Anyway, 1 >>>>> 2 and 3. With no other slingers/scoundrels in raid, df can do wonders
1 is mostly a parsing issue, though, and not losing much raid-wide dps, right? As in, you're contributing extra dps to someone else's parse while losing it from yours. (Admittedly, it's likely their dots and not their wounding shots that is eating up the stacks, which is somewhat worse.)
[I'm aware this is a parsing thread, not a raid-wide dps thread.