Imagine you're browsing a feed and your client fetching metadata for all those people on the fly. You can either fetch an event for each or you can fetch 20 events for each and reduce them to a metadata object by applying diffs locally. Do you really think the second is the best solution in the real world?

Also, since you're just fetching random people's profiles you cannot ever be sure (or how could you be?) that their sequence of diffs is complete. You would actually need a blockchain to be sure.