I still feel a need to tag projects to make the process of setting up and maintaining my perspectives a bit easier and to make it easier to find certain projects that honestly can get buried.
OmniFocus has always been a powerful "lump of clay" with the ability to configured to one's own process - I think tags would be a welcome feature leaving it up to the users whether or not they wish to take advantage of it.
I've used OmniFocus every day for two and a half months. The more I use it, the more I want tags. Every app that can be described as a "powerful 'lump of clay'" has features that some people think are essential, while other people never need use those features. For some of us, tags would elevate the usefulness of OmniFocus tremendously.