I think differently. Let's take the organic groups module case. It has the core functions, but for lists it uses the Views module. In other words - you don't have to install Views, but its is recommended.

I'll project this to rules and CA. You may install ubercart, but for triggering actions you will need to install Rules.

Fago is a well known contributor, having a dependency on one of his modules IMHO is safe.

Anyway, as I suggested before, I'll help with any Ubercart's decision regarding this.