Yes.

This kind of technique, and the exploitation of minor CPU errata, can be used to help differentiate processor models and steppings.

That in turn allows a currently widespread DRM system to download personalised portions of object code that rely on properties specific to the licensed hardware in order to execute properly, in an attempt to counter debugging, emulation and transfer - continuing a tradition practised in copy protection techniques since at least the 6502, maybe even earlier.

Is there a term for that, similar to "security by obscurity"?

how does "Identification by exploitation" sound.

I would love to know the answer to that. Something tells me that we need to fix disassemblers before we can answer it though.

Do they have any benefit over the valid instructions?

Has happened in the past...

http://www.rcollins.org/secrets/opcodes/SALC.html