Rana:

So - what's the metric for calling a middle relay "useful"? Is it the total number of bytes that it relays daily? https://atlas.torproject.org/#details/707A9A3358E0D8653089AF32A097570A96400C C6 is sending about 0.85 GB every 24 hours. Is it a "useful" relay?

Sure it is! If there were only blazing fast relays it would decrease anonymity because these relays would be placed in some datacenters and operated by small amount of people (entities).

Tor network needs all kinds of relays to be strong. Diversity is about platform, location, connectivity, etc, etc.

If you think that your relay is underrated or has poor performance try to adjust your hardware/settings. Anyway almost every relay operator has this kind of "operator anxiety". Don't worry. ;) -- Ivan Markin