Khh is way stronger then burning attack, look at charge times and how burning attack differs from nearly every other beam attack in the game (blinding people, sure you could say the same about solarflare, but solarflare costs more ki).
Burning attack is not crap, it's just not ment tobe used as a beam for power struggles, it's more of a tactical thing since trunks is more of a melee charcter more then anything, he's also getting the flame thrower (the move he used to finish off frezia after he sliced him in half). There's also been suggestions of adding buster cannon or finishing buster before (finishing buster as abeam or buster cnnon, I don't remeber it exactly but there is a thread on it).