That's now becoming an important question for those certain of wanting to get the full CPL/IR.
As someone said, FTO's will probably argue in favour of the IMC. So I'll announce my personal interest and let you know that I run an FTO so I may be biased.
I think the answer to your question all depends on how you intend to hour build and when you intend to take the CPL flying training.
If you want to hour build in the UK as quickly as possible, then I'd say an IMC would be worth having - a) because you can fly in the poorer weather days and b) because you can practice your I/F for the CPL. If you intend to go to the US or somewhere like that, then there's no point having an IMC.
If you are building the 100hrs P1 time with the intention of going straight on to the CPL, then again I'd say it might be worth considering an IMC. There's a reasonable chunk of I/F in the CPL flight test and the more comfortable you are with it, then the easier you'll find the test. If you actually have a number of hours then maybe the IMC isn't so critical as you should already be able to handle the aircraft comfortably.
As I said, I'm probably biased, but my rule of thumb has always been to get the training done using the cheapest instructor available. If you think there's a risk of over-running on the required hours for the CPL or IR and paying £400 per hour for the pleasure, you might find that having the additional I/F experience at say £130 per hour actually reduces that risk and works out to be the cheaper option. What's more failing CPLs and IRs is a very expensive business, so I'm all for minimising that risk as much as possible - and an IMC would do this.
PS. I agree the system is ridiculous. Why shouldn't you get an exemption with an IMC rating? Anyway, you don't; so ask around and see what the general view is.