I agree that there could be particular circumstances where EWI can be applied selectively. As Eon have now completed the technical survey for my house, I expect them (or rather, live in hope!) to confirm a start date shortly. However, like you, I am wary of agreeing to having all of the external walls clad.
The problem that I found is that it is not easy to reach a human being, as (surprise, surprise!) they appear to be taking a tickbox approach to the process.
I also do not understand why, unlike the earlier (failed) central govt scheme, internal wall insulation is not an available option - For a mid-terrace house like mine (south facing and almost opening on to the street), IWI in the master bedroom and in the front room would arguably be 80%+ as effective and, the house already in a state of renovation, would cost them less to install.
Anyway, I don’t know if any of this experience will be useful to you, but it at least cathartic for me get it down!