That's not what I mean. I'm sure the strawhats WILL defeat Buster Call. But for it to just be wiped out so easily as to say, "Oh, Buster Call was no big deal", that would seem silly. And this is of greater caliber to an extent, considering that it's not one person, but several like Ao Kiji who Luffy was no match for the first time around.
There was nobody on Ohara except Saulo who even stood a chance to defend it. On one hand you can say that goes to show that it's not that strong, but on the other hand,you can say that we don't really know how effective a large scale challege could be. As someone pointed out, Saulo, an admiral and giant, was not able to defeat Buster Call.
Saul was doing pretty well against the BC ships until Aokiji took him out of commission - it wasn't the cannon fire that did him in. In any case, when I speak of Usopp defeating the Buster Call what I mean is him sinking the ships preventing EL and thusly the mugiwaras from being burnt to the ground by cannon fire. I don't mean Usopp taking on the Vice Admirals by himself from close range.
Now the VA's may or may not prove to be a problem for the mugiwaras, but their threat can be neutralized as long as Usopp can keep them from coming ashore with his sniper fire. If a VA does manage to make it ashore, there are still the other mugiwaras who can contend with him/her. They can keep the VA busy while Usopp sinks the rest of the ships from a safe distance.