1. you can use the opcode, it's guaranteed. if it does something else, that's not on you.
2. you might know everything about the environments and that's how you know that different environments use different interrupt tables, but you want a code snippet that will not rely on that, so you use this guaranteed opcode.
Usually, you do know something about the environment that your code runs in. But in the case that you don't, causing an exception (whether "undefined opcode" or anything else) can't be guaranteed to terminate the process, because some operating systems actually did use it for other purposes. The assembly language equivalent of nasal demons :)
2. you might know everything about the environments and that's how you know that different environments use different interrupt tables, but you want a code snippet that will not rely on that, so you use this guaranteed opcode.