> I don't know where people would run three versions at the same time.

Not on the same machine, but server/automatic -> GnuPG 1, desktop 2.0 or 2.1. Also different people may run different versions, even GnuPG 1 on desktop because they are used to. This compatibility mess that seems to persist for while was what I meant why people prefer to use the lowest common denominator in their sigs/cards/slides -> evil32.

> What 3rd party software is using the keystorage mechanisms directly?

Aren't there any? Good, then I misunderstood that argument in the Homebrew debates I read, sorry. That leaves only the fear of automatic upgrading and the inability to downgrade again as a blocker.