without one clients will not be able to look up user IDs using 3PIDs, so its relatively important. hosting it yourself means having to email or message "us" in order to federate it.

So don't use a 3PID to add someone, use their MXID. It's a unique, global identifier and is similar enough to an email address.

There's also a Matrix URI specification in the works (https://github.com/matrix-org/matrix-doc/pull/2312). Once that lands, we should start seeing support for Matrix URIs in applications. This will allow applications to also start recognising MXIDs and converting them into working links, which will in turn help with people's recognition of the MXID.

I've been using Matrix for three years I believe and I can't remember a single time when I needed to use 3PIDs.

does matrix store anything in the server or is this really p2p? can this be a good replacement for say, telegram?