hi
firstly for the record I am a freelance construction/project manager so i may be slightly biased

- but personally i have been on both sides of the fence on this issue
one of the jobs i am currently managing is a project where the architect did the design & planning submission and left the project (by prior agreement with the client - no falling out or anything!)
as per the prev posters - you do not necessarily need the original architect for the build nor do you need an architect per se but IMHO you will need a professional "consultant" to oversee stage payments and planning compliance/building control etc if they are requirements and to be on hand to advise you if you are not experienced in construction
it is handy to have the designer on hand for queries as in my experience its seldom easy to realise a design fully to the clients satisfaction from drawings only, no matter how good they are. particularly if the architect has had a creative input based on the clients ideas which may not be described in so many words on the drawing
the builder will need someone to ask about the finer elements and in the absence of anyone else will either look for an answer from the client ;who may not have the time, inclination or knowledge to answer; or make up the answer himself - in either case the client may not end up getting the end product they desire
i would be concerned about giving the contractor the role of "project manager" and i would be interested in seeing how that could be covered by a clause in a traditional domestic building contract (especially if one provided by the builder?) - by all means they should be responsible for the construction management eg the site works etc and the scheduling but IMHO the project manager should ....
1.have a view of the bigger picture and the end-to-end project not just the construction
2.be acting on behalf of the client with no conflict of interest, while upholding the terms of the contract
the builder should at contract stage provide a fairly comprehensive list of things that he will be responsible for dealing with and things he won't - if the client is not confident in his/her ability to provide the latter, then obviously a third party is required.
again IMHO projects of this nature go off track because the builder has an idea of what the client wants of him and the client has an idea of what the builder is going to do ......... but most times they are not the same idea and this lack of understanding will always cost one or both parties time and/or money.
best of luck
M