Hey,
I agree with JHT_Seattle that you would need to rebuild all if you start using writable volumes due to the fact that it is stored into the writable volume entirely. There are some differences though in using writable volumes and roaming profile. Roaming priofile only holds the roaming parts of the appdata, writable volumes hold (for as far as i know, check the cfg file if you want to be sure) all parts of the profile, also the local. There are quite some applications that input info into their local profile parts (think of Mozilla FireFox settings and i believe Chrome does the same thing).
I disagree on the UIA parts of your discussion. We have multiple purposes for them, 1 being giving a user the options to have applications he wanst to install himself (without us having to woory about support) and still be able to use it whenever where-ever.
Also, i someone needs a "test machine" you can give them a writabel volume, attach it to a machine, let the user fiddle around with it and when done just remove the writable volume. No need to create extra machines, all can be done with 1 golden image.
I must say that our users do have permissions to install applications locally so for us it does have a business case..