First, I completely agree that there is a limit and it is up to the instructor to judge what training the student needs to accomplish the training objective. With that said, a partial panel hold can be part of an instrument approach procedure. Even if 999 out of 1000 times that you could possibly have an instrument failure you will be in RADAR and radio contact, that one time when you're not, you need to be able to deal with it.

There is precedent for being able to fly without ATC assistance. Our entire instrument system was developed from, and continues to have procedures for, a non-RADAR environment. ZLA has lost RADAR numerous times and radio a few times. SoCal TRACON has lost RADAR and radio multiple times. ZOA and ZFW are not immune. Whatever is causing the failure can be causing multiple failures. A short-circuit or fire can cause a progressive failure. So can a engine that is spewing fuel out of a major leak.

Now, myself, a few clients, and two instructor friends have experienced instrument failures while IMC. Four times, ATC RADAR was unavailable, twice, RADAR and Radio was not available, the controller stating "call me when you land to cancel IFR". I've been in IMC and lost pretty much everything except the radio (two FAA ramp checks followed that flight, the first as the Inspector happened to be on final when I came in, the second 'cause he called his boss). Hopefully you will be one of the majority of pilots that never experiences a serious in-flight failure. We train so that if you are one of the "chosen few" that will be put to the test, you will survive.