Apart from preparing you for the IR, two other points:
- You need to do some hour-building before you can start your CPL. The IMC rating can count towards this hour-building. Be careful, because a lot of the hour-building has to be P1, and if you have too much dual time you risk having to fly more than necessary to get your P1 time up - but you can still do the IMC rating without having to log any extra hours if you're careful.
- Although the IMC rating doesn't count towards the hours required for the JAA IR, it does count towards an FAA IR. If you get an IMC rating, and add a few extra instrument hours during your solo hour-building time, you can get an FAA IR with just a few (my memory must be going, I can't remember how many - I think it's 15 or 20) hours with an FAA CFII. You can then convert this to a JAR IR with 15 hours of additional training. If you take this route, the total time to a JAR IR can be reduced by a few hours.
As for how relevant it is, I think everything you do on your IMC rating will be relevant to the IR. You won't do any airways stuff, but you will learn the basics of instrument flying, and you will learn to fly approaches, albeit with much higher minima than for the IR. (Although if you get an understanding IMC instructor, there's no reason why you can't practice your instrument approaches right down to IR minima during your training).
FFF
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