hawker750, have you considered, a route could have differing track on route legs which means differing ground speeds? Differing winds at flight levels (ground speed again) and temperatures (TAS this time - excuse the pun) during a climb or descent?
The current FMS's do a very good job on the RTA front. I particularly like not having to use a abacus after a 14hr flight to Australia for instance, where inevitably an RTA is issued mid descent.