kde-base/krdc-4.2.2 has the following automagic RDEPEND: libvncclient.so.0 => net-libs/libvncserver:0 which is not part of the dependencies Reproducible: Always
In case it's related to this "automagic" stuff, an actual problem is that ~krdc-4 does not depend on or have a USE flag for rdesktop, which is QUITE necessary.
The libvncserver dep is listed on krdc-4.2.4 - let's assume it "was fixed". I've added a postinst warning about the need to merge rdesktop to establish RDP connections with krdc.