This definitely should be considered as a utility function like GetEntityVectorOrigin or something.
Because I was a newb and tried to use GetClientAbsOrigin or GetClientEyeOrigin to get the origin of a witch (L4D) and it kept giving me errors saying it wasn't a valid client index!
I mean sure it wasn't a valid client index, I just couldn't figure out a way till CrimsonGT suggested using the m_vecOrigin netprop.
At least TeleportEntity works on Entities
.