If you name a priority as a context though True Believers of GTD wouldn't have any objection to it.
Gtd does use priorities, it just names them differently. Someday/maybe is a list for low priority stuff. Also think about a "surf" context. It's basically a context for low priority computer actions.
Priority is just another attribute and a way to group similar actions, it's essentially exactly the same thing as context.
Why browse through 100 actions each time when only 5 are important? Why not just flag them and save time and effort. You're not going to lose anything. In fact you're just saving time.
The whole point of gtd is in externalizing your thoughts. So if you really think that something is high priority you might as well externalize that information. If everything in your life is roughly the same priority then you don't need it.
Same thing with contexts btw. If you spend all almost all of your time in the same context then contexts aren't very useful. But if you switch between contexts a lot then they're useful.