One really big advantage to moving to .adp's that validate
as xml is that there are a number of bugs that are not
diagnosed very well when the adp is malformed (eg
an unclosed multiple tag produces a not very intuitive error).
Having the adp's validate would make automated testing
for that sort of error straightforward.
On the other hand, dreamweaver will work just
fine even if they are not well formed xml (although maybe
this has changed since version 4 which was the last time
I worked with designers who used it).
I think it would be pretty straightforward to fix
the most common things (singleton tags, attribute quoting,
and case would be most of it),
but <if> tags and the nesting of adp tags inside html
tags (like on the body tag in the master template)
would be problematic.
quoting everything and fixing case can be done w/o
discussion, but we would have to have a TIP for the
other required changes and while I would vote yes for
such a change I am not sure other people would.