Titles are useless, until they aren't.

Titles for the purpose of power is bad in a collaborative work. Title for the purpose of communicating knowledge are pretty damn useful because they are a shorthand.

same goes for "senior" IMO because its definition depends on the company and is different everywhere.

The meaning of junior, medior and senior varies wildly by company. They're practically meaningless.

Medior?

That's apparently in between junior and senior. Some companies use it.