I cannot imagine a real-life legitimate situation when someone may create a file with FILE_SHARE_WRITE access while performing a write operation into it. I truly can't think of a single one, can you?
Seems we're digressed alot, but yes, I can imagine that several apps write to the same file "simultaneously", or better say, opened the same file for writing. This is only the matter of design, same as use exclusive access or not. The fact itself that such "shared write" access flag exists (and not only in Windows ) tells me that I'm not the only person who can imgine such situation.