Boingy, my key phrase was "if you have to do the hours anyway". Unless you don't pay for your hour building then the additional cost is very much less than that you quote.
In addition to teaching you skills that will come in useful for the instrument portion of the CPL and help you during the early phases of your IR course, a good instructor will pick up on and address any bad habits you've adopted since PPL. He'll end details VMC with simulated emergencies, he'll pull you up on your log keeping, he'll nominate your touch down points. Sure, it will cost you instructor time at 15 x £20 plus a dozen approach fees but what's that, one hour in a twin at commercial rates? Half a CPL re-test fee?
And if it makes you a safer pilot during the rest of your hour building then even better.
Just an opinion based on my own experience.
edit - I do agree with you about the challenging XCs though. Despite spending 15 hours on the IMC and 6 on the night qulaification, I still managed to do plenty, including 50 hours solo around and across Oz. The trick, IMO, is to do a mix of things, building experience AND skill as you go.