Alright, I just kind of skimmed through the other replies.
Boring - Sure, it's boring, but there a ton of "boring" categories out there. You have a point, BruceS, when you say that just because we have some doesn't mean we need more.
Prevalence - I agree with many of the comments about post office locations having post office boxes. After some thought, I think those should be exluded as it kind of goes without saying that post offices will have the blue boxes. The other part of the prevalence issue is that there are so many. While this is true, I think again to the payphones. At every 7-11 I can think of in town here, there are two payphones right outside. Should each one count as it's own waymark? As far as I can tell, the category will allow it (based on the cat description). In the USPS blue boxes cat I have in mind, a grouping of two or three boxes would count simply as one waymark.
To sum up my arguments:
Global - No, but there is no need for it to be. I believe that this would work best in a framework similar to historical markers where each region (post service area) would get its own category.
Prevalence - I believe by excluding post offices and having groups of boxes count as one waymark, the number will be greatly diminished.
Interesting or Informative - I will not debate whether or not it is interesting. I think we can all agree that this is a downright boring category, with the occasional interesting waymark. That being said, however, it is an extremely useful category, such as payphones. Variables would include things of use to visitors, such as pickup times.
Redundant - Hardly. Since post office locations would not be included, no cross posting would occur there. There would be a few cross posts from other categories (such as All Things Star Wars), but those are going to be rare.