I think GW rationale went like this:
"Hey, let WField be the exact same as ExtCar on a Tyrant! Surely most people won't bother gluing on a few pieces of extra carapace, when they can get the same thing for a 'few' points more and
no modeling!"
Sorry, GW, but my modelling skills are good enough to glue those few extra carapace pieces on and save a load of points I can use to make my Tyrant more lethal!
GW rationale, part two:
"Hey, if someone wants to take a fully biomorphed Tyrant, they can't do it since there are 9 biomorphs! So, they'll have to take WField to be able to drop ExtCar and take all the other 'morphs!"
Sorry GW, but I don't really think Flesh Hooks are too smart a choice on a Winged Tyrant... So if I
really want an über-Tyrant, I rather drop the FH and keep everything else, including the rather cheap ExtCar!
So there You have it!