...if enough people want priorities, then what's the harm? If you don't want to use them, don't, they'll be invisible to you.
Every minute spent on adding priorities is a minute lost fixing or developing some other more important or more appropriate feature. And, at least in my mind, there are many things I'd much rather see than support for priorities.
The risks are significant: increased design/engineering cost, increased complexity, straying from canonical GTD, pollution of the UI, etc. And the benefits are negligible.