@Graham *****
It's not. Cambodia stopped accepting them except for residents of border districts and then they have significant limitations on travel. Passports are mandatory for travel to Siem Reap.
Given how easy it is to apply for one, it's ludicrous why anyone would use a border pass, which, in the places they're accepted take time to issue and cost money. With a passport you can just travel directly to the border crossing and travel around the neighboring country.