The flaw of a weak story is more apparent if you try to explain it. Nobody asked why Cloud Strife and Sephiroth showed up in Kingdom Hearts. They're literally just there (Cloud in theory made a deal with Hades, but it's not like anyone really cares) and that's probably the best explanation. Further, both figures are legendary figures in their genre so their presence masks whatever flaws in the story to begin with.

Lightning, on the other hand, is a relatively lightweight, but for some reason SE seems to think she's already more populated than Cloud and Sephiroth put together.