NetworkManager allows the dhcp client to be toggled at runtime via NetworkManager.conf. There is no need to enforce an "exactly-one-of" (^^) relationship via REQUIRED_USE. Given bug 565108, I would propose that this REQUIRED_USE constraint be dropped entirely. ^^ ( dhclient dhcpcd )
Personally I would even disable completely the unmaintained dhcpcd support to prevent people to enter into bugs that won't be resolved ever :/ Maybe for 1.2.x cycle we can make all that dhcp* changes
I tend to agree with comment #0. Since it can be configured at runtime, I think there's no need for the required_use setting.
dropped the restriction in 1.2