These packages make use of gtk3 useflag which indicates that there are issues of migration to gtk+:3.
Gtk3 and similar useflags should be avoided.
Adding to tracker. Please have a look.
The issue with using gtk3 for browsers is that it breaks the adobe-flash plugin since that links against gtk2.
Therefore, I think it's useful to offer the option between gtk3 and gtk2 in this case because some people don't care about adobe-flash while others need it to work.
Closing as won't fix as discussed in comment #1.
If this is a big deal we can rename the use flag, but the option should remain as long as official flash support requires gtk+-2.