emerge-ng and vcs builds

gentoo is a source based distribution, and this is a good thing. why then has the ability to have trunk builds (which has been added to many overlay's) seem like such a hack? all non-binary program ebuilds in the tree should have a 'trunk' version option. emerge --sync should then be able to find out whether trunk has been updated so you can correctly rebuild programs as needed with emerge --ask --verbose --update --deep --newuse world