The most important thing is currency. I have an IMC rating, and use it on almost every flight I undertake. This may be for cloud break, an instrument approach, VFR on top, or simply flying instruments for the extra precision it offers. It is great fun, and even for flying VFR in limited visibility (where you are "legally" VFR, but in practice on instruments) it is incredibly useful. I'm not sure how I ever managed without it. And, it all counts towards the CBM IR. I can't think of a reason not to do it.