[tor-talk] Why is 'Wgm' (middle-relay-for-guard weight) not zero?

[starlight at binnacle.cx]

The recent major fix for #17772 inspired
reflection on the practical effect
considering relay weighting.  Knowing
nothing about it aside from the graph
lines shown by Atlas and Globe, I thought
the change might make little difference
as Guard Weight would mostly prevent
non-guards from being considered.

However, per dir-spec and the current
consensus, it seems that middle relays
have a weight equal to guard relays
when guard selection occurs:

Wgd=0     - exit as guard
Wgg=6065  - guard as guard
Wgm=6065  - no-flag as guard

I would like to understand the purpose
behind this.  Can anyone comment?

Thanks