There is almost no case for *you* to 'use' simplex, although if you are already using a server for internal business or have a node its not a terrible idea for messaging. Simplex offers more privacy for DMs, the push is to integrate simplex into your "nostr app" so you don't even have to think about it, when you send a DM it will be sent over simplex rather than nostr to preserve your privacy. Nostr is a great tool, but I am skeptical that it can solve all the problems on its own, better would be integrating different protocols better fit to purpose, just like lightning is integrated now.

Why people suddenly want to jump on simplex as a standalone client, I don't really know, I use it for support for graphene OS when I am having trouble but its not where I would hang out, unless I wanted a more discord style chat server.