That is what their role really is. To basically take one (or two) specific player(s) out of the equation. These players are usually always the healers and occasionally a high-powered dps that could do massive damage if left alone. That's another example of how the charts don't tell the whole story. The damage numbers a scrapper may have done will usually be significantly lower than his peers, however he made a major contribution by shutting down the enemy team's healer.
As for concealments, i agree, question is, isn't the assassin better at this? I see in WZs ridiculously high numbers of them, while Concealment ops? 14ppl out of 800 ... And Lethality ... well noone play them (with a few exceptions) because of bad design.
Thing is, if you play Lethality, you're just hurting your team, because of looong, looong duration of dots (preventing any CC), which is (as the game is now) major problem (besides other purely technical problems - bad design)