There are 4 main usecases: 1) for a user who wants to have a gtk:3 only enviroment for example full gnome:3 desktop, 2) for a user with xfce-oriented desktop, 3) or a user with a couple of gtk dependent applications. (again to have clean gtk:2 ot :3 enviroment) 4) Gtk 2 nad 3 have quite different styles, so one can prefer either of these libraries to have the same look among applications. Reproducible: Always Expected Results: To have useflags to choose between gtk versions: gtk and gtk3 for example.
Packages that allow such chois, e.g. x11-terms/evilvte x11-terms/roxterm app-editors/gobby x11-libs/vte media-sound/deadbeef and even more
No. GTK+-3 is an upgrade, not alternative. Using bug 374057 as reference. *** This bug has been marked as a duplicate of bug 374057 ***