The other issue that comes into play is that in order for someone to earn your trust, you have to give it to them in the first place ... when they consistently follow through, then you can be comfortable (or not) with how much control is needed on tasks delegated.
In my particular case, working from a remote location, I rely on administrative help in our home office. Unfortunately, the turnover for those positions is high, and it may not always be the same staff handling requests.
I would love it if our organization was committed to following GTD principles, or heck, any principles at all, from the top down. But in a large, 1000+ employee company, everyone has their own methods (or not). For this reason, even the 'delegate through Outlook' method would not work.