I think it was around 1999 when a young girl picked up a metal object on Great Mis Tor thinking it was a letterbox and it turned out to be a grenade or something. After that there was a lot of controversy about using ammo boxes. At the end of the day, the army should be more careful and pick up their rubbish. (I still find army junk scattered over the North Moor).
I used ammo boxes for years, they are perfect for letterboxes and I don't think they have ever been bettered - and they are easier to find than pill boxes!
After that incident with the girl on Mis Tor there was talk of Park Rangers and the Army removing letterboxes housed in ammo boxes (if they came across them) and taking them to the offices in Park. I think many of my boxes disappeared in this way, but I never found out how true these rumours were. Are many people still using them? If so, I might return to them again - I have recently discovered the Otterbox..
http://www.psalmgear.co.uk/Otterboxrange.htm
..which seem to be ideal...but very pricey (especially if your box gets nicked.)
Like someone else said before - if the ammo boxes are painted to remove the army decals I personally don't see a problem.