[jaemoe]

I see the only use of 3PIDs in a professional setting where people have to find co-workers using emails or internal phone numbers.

In my opinion, it is really useless for anyone else.

[decentralisedog]

I've never used the feature on Matrix because as you say, it doesn't really apply. Matrix (or really Element here) doesn't require you to use an identity server at all, so I don't really think it invalidates the protocol in a meaningful way.