There are some cases where it would be impossible for a package to co-exist with multiple versions. For example:

Package A creates a GUI window and provides functions interacting with it.
Packages B and C imports A/v1 and A/v2 respectively.
Package D or a executable uses A, B and C.

Lets say package A stores the window handle in a package private var, so either package B or C will work and the other fail.