It's a good way to get at least a ballpark on how powerful they are. If someone destroys an entire city block as a side effect after merely a single lazy swipe with his arm, we can at least assume that if he decided to put in any real effort it would wind up being significantly more powerful then city block destroying.
It's the same even as you go into higher levels of destruction. A character having to pour every single bit of his power into an attack in order to be able to destroy a planet isn't anywhere near the same as someone who destroys a planet without even specifically meaning to do so. We can't get an exact measure on how far above planet busting they'd be, but it'd be a safe bet to assume "a good deal above it".
Though you illustrate a perfect point, if a villain destroys a few universes as a side effect of a fight is his power multiversal? Well, for instance, what if a villain destroys a few universes as a side effect..but we also know that there exist thousands if not millions of individual universes inside even a single multiverse? Calling him multiversal might be technically correct while at the same time might also make him seem more powerful then he really is. Since destroying a few universes is impressive, but it's not even a blip on the radar if there are millions of universes in the multiverse.