Consider the following recommendation, taken from the official FAQ :
You should set
MyFamilyif you have administrative control of the computers or of their network, even if they're not all in the same geographic location.
Now let's say I operate two relays and apart from who pays the bills, those have nothing in common: they run different OSes on different hardware and are hosted by different ISPs in different countries. Heck, let's even say that they don't run the same Tor version.
Let's also say that I omitted to set the
MyFamily field and consider the rare event where a user selects a circuit that involves both my relays.
a bad guy (that is, I monitor all traffic passing through), then that's cool because that'll make traffic analysis orders of magnitude easier;
a good guy, then that's cool because I'm a good guy and I know my relays are clean so the more of my machines that channel involves, the safer it is ultimately.
So the only useful use case I see for the family property is for a bunch of relays that somehow share the same infrastructure, not the same admin. I know I'm probably wrong thinking this, so:
Why would I want to set this field?