While it's true you can reach places others can't with a BMX, you can easily find another BMX to get where they are, or even dive a helicopter into the area. There are many options to consider when it comes to that argument. In fact, if the argument of why BMX's shouldn't be allowed for wanteds is based on where they can get, then NRGs/bikes should be prohibited as well.
However, I do not support this amendment to the rules, as the BMX remains indestructible, and such, the only way for a cop to get a wanted on a BMX is to kill him. Disabling and arresting should be an option, and the BMX's invulnerability, quite simply, nullifies it.