Daryl is pretty much the only character who could really take over for Rick in the short-term, and Rick has become increasingly unstable. None of the other characters really have the potential to keep the group together. That's probably one of the biggest reasons why he's indispensable right now.
Merle could take over in a pinch if we're strictly dealing with a fight, but he would have to step aside right after the fighting finished. Also, there's really no way he could take such a role in the group without Daryl and (probably) Rick being around.