they dont belong plain and simple. Devil Jin is an annoying creature.
i never understood putting in a character with no weapon when there are plenty of weapon using characters in other series. i think even Valeria's kick edges from SC3 were bothersome.
there are still weapons not used yet in the SC universe and tons of unique styles to play with.
...but to make a weaponless fighter work in SC i think itd be cool for he or she to be like Makoto in SSF4 with short, quick, controlled dash attacks and then have alot of evasion moves. its more logical and fair than an annoying rushdown character that even noobs can use and win due to superior frames.