Well, if Multi-fighting was re-implemented it'd have to change. Before when more than one person was interlocked with you/was free-firing, you had a penalty. Multi-fighting negated that penalty for up to three people I believe.
Multifighting - activates while you're interlocked - gives a defense bonus against out-of-interlock attacks. Sharpshooter - always active except in interlock - gives a ballistic accuracy bonus outside of interlock.
This is confused
Multi-Fighting
Sharpshooter
I think the description given for Sharpshooter is on the right track. Logically, there should be a penalty of some sort (presumably an accuracy penalty) for shooting at a target that's interlocked and Sharpshooter could be ysed to negate that penalty. Multifighting would act as a defensive buff against any attack coming from outside of interlock (or, as I've suggested before, at least reduce the chance of outside attackers being able to cause the special move enabling states like staggered). This way, the two abilities wouldn't completely cancel eachother out and end up making any changes nearly pointless.
Anyway, as said, either do something with them (the much better option) or get rid of them altogether. This is one of the most commonly asked questions that I hear from both new players and those who have returned after a lengthy time away. They always ask what they do and then, once I tell them they do nothing, they ask why they are still there.