OmniFocus can be used to implement the "Getting Things DoneŽ" work-life management method developed by David Allen, but it's designed to be flexible enough to accommodate different styles of personal organization.
Arguments about prioritizing in GTD aside, OF is designed to be useful even if you don't use GTD. As long as it doesn't force a user to do things in a non-GTD way, I don't see what harm is done by including non-GTD concepts.
Specifically on prioritization, I don't think it's anti-GTD at all. GTD helps you define what you can do at any moment, and I think the ability to prioritize those results is useful. It would be best implemented as a way to sort the results of your filtering, and not as a filter setting itself.
If I'm making phone calls, and have 3 calls to make: 1) I can call to reschedule an August doctor appointment, and it's due by the end of July while they still have appointments available. 2) I can call Payflex about submitting hospital expenses for reimbursement, also due by the end of July. Or 3) I can call my friend Steven for a chat since we haven't talked for a while, and I should call him in July because he's a teacher and school starts up in August again.
Three calls, all 'next actions', all are 'due' by the end of the month, and I have the time, energy, and context for making all 3, so which do I make?
I should make the Payflex call first, it's hands down the most important. But if I have a long list of calls to make (and the phone call to Steven can easily be multiplied by 20, I'm notoriously bad at keeping in touch with people), I would like to be confident that I'm doing one of the most important things available for doing.