I think that the thing payphones has going for it is that they are dying at a far more rapid rate than blue boxes ever would (unless the post office went away). I don't particularly mind payphones, no they aren't exciting, but they are becoming more and more rare every year thanks to the cell phone. While I personally would like something more intriguing, I do think there is a place on waymarking.com for rarities.
If you tried to take blueboxes off of the post offices, I'd still be torn. I do think they are becoming more and more attached to post offices than they used to be, but I don't think they are that uncommon. You'd also need to specify how far away from post offices it needs to be. I know of one particular post office box that's about a block away from a post office on a more main road for the convenience of drivers. Yet I would still consider this a post office blue box and I could totally see this type of thing going through because well it isn't right next to the post office.