I don't think that posting on wiki has the same effect, because you can see that it's a discussion forum. You would probably hope that your report will kick off a chain of events that leads to your desired change being made, but you aren't likely to think that what you type (beyond the natural results of the facts) would directly change how quickly the problem is addressed. That is, if you post on wiki saying "This is broken, please help", you expect the same initial results as if you post "This is broken" or "This is broken and very important".
If we could "semi-protect" the priority field, people would see that they can't change the priority (directly). They can't change the words on the page and they can't change the reality. The appearance and the reality match. Looking at it from a design-ish perspective, if I can't change the reality, why should I be able to change the appearance? Why would someone bother installing an accelerator pedal (or a brake pedal) for passengers in a car, if the pedal doesn't have any effect on how fast the car is moving?
We lose transparency if we hide the information. My point is that the current system is also not transparent: the system falsely tells users that they can change the priority, even if they can't affect it at all.