thats what i try to do, keep the model as fluid as possible until i'm happy. thatway i can toss or pop in new geometry later. what sucks is when you finalize it, think your done, look it over and discover that A) you forgot something, or B) you came up with more ideas to add on to it. thats when it gets to be not so easy.
as for a roll, your thinking sort of like a piloted mobile point defense platform, or a bomber support fighter. not a bad idea. in a way thats sort of how i intended the wraith to be used, mainly assault and bomber support though. but i do like the idea of the defender, kind of an odd twist of interceptor, but not. intseat of chasing you down personally, they chase you down with long range angry missiles by the airtight boat full.