because one cannot generally rent planes suitable or even legal for IFR
A bit of a sweeping statement, IO540. Maybe many are not really
suitable for IFR, as so few have decent modern radios, for example, but most of these flights will be outside CAS so will be perfectly legal in your average flying school junkheap - remember that the legal requirements for IFR in IMC outside CAS are minimal (IIRC not even an AI is required).
Totally agree about the IMCR being useful only if you can fly an IAP, though - otherwise what happens when you enter cloud and then find there's no way out when you thought there was? Hence the usefulness of the FAA book (and the Instrument Procedures Handbook, which I also found very useful reading).
Tim