Apart from the issue of protection, I like pipe for two reasons: Changes and cheaters!
Pipe allows you to later upgrade the system by pulling new wire. Since I hate digging, using pipe meand never having to dig again.
We have rules, as well, regarding depth of burial and the use of "clean fill." In my experience, however, the folks who use direct-burial cable rarely place the cable more than a few inches deep, and are quite likely to lay rocks right atop the cable!
So, from my point of view, I've come to associate direct burial cable with lousy installations.
