Tom,
The goal, I think is to allow reusable forms. Allowing ad_page_contract to be used on includable forms is useful.
I think fixing a shortcoming in ad_page_contract is not a bad thing. I definitely think that it can be accomplished without breaking any existing code.
We have to remember that ACS 4 that OpenACS is based on was never finished, and there will definitely be places where we can improve on what we started with. As more and more developers use OpenACS to built real web sites, they will find things that can be improved.
ad_page_contract also serves to provide consistent documentation of tcl/adp pages. I think just that feature is worth using on includable pages, besides the other benefits.