Keep in mind that you will do quite a bit of instrument training during your CPL, so if you are planning on doing the CPL before the IR then maybe an IMCR will not add much. You do need to be pretty comfortable with instruments in the CPL skills test. Granted, there are the approaches and holds, but as an IMC holder I'm not convinced I got much from the IMC as there are a number of differences in the techniques so some unlearning might be required. A clean sheet start in the IR might actually be a benefit.
If you are thinking about using the IMC during hour building, then consider that the majority of IMC holders have not used the rating in anger, so without knowing anything about you I would venture that statistically at least, niether will you so there is a possibility that it will just be more unecessary expense.
With regard to other arguments that you learn to fly more accurately my experience, as one who is no more than average, that my ability to fly and plan accurately and efficiently is related more to my mental attitude and not really from any extra ratings I've collected along the way.