" It is somewhat useful, yes, but can you compare to most of other ultimates around?"

Sure: Most races are defined by their ulti, Raps is one of them. Some are supporting their others skills (Warden) or are off the wall with the rest of the character (Blood Mage).

Also, "that KDR means jack" = "that KDR means jack shit". Not talking about the race.

" Raps crit < setting a person on fire"

I might accept setting on fire + weapon drop. Just setting on fire only happens (just over) 1 time in 10 more often. (22 vs 35) If you feel that slowing them so you might kill them is better than straight up killing them, with such a low difference on proc rates, then we're going to have to just agree to disagree.