Part of it is wishful thinking. Do you ever want your kabalites in melee? No? Then why assume they'll end up there?
They are characters, so precision shot is a thing, but most people don't think that giving a Sybarite a Splinter Cannon is kosher.
Mostly Sybarites make alright challenge monkies or leadership buffs to large squads as has been mentioned above.
I think an argument can be made for the Dracon as a clean-up character. That is, as a character meant to assault into squads of 2-3 marines. Possibly with an offbeat choice like shredderborn.
Time for the grateful DEldar.