There is no restriction on doing your CPL Skill Test on a multi - you avoid having to do PFLs and glide circuits but have the asymmetric section to cope with instead; the bit about having to work harder on the navex is a red herring as you will be expected to fly a longer route. You will get a MEP class rating in your licence on the basis of this test. At least 2 of the Integrated schools now do their PT3 (same as CPL Skill Test) on multis to avoid having to train on a complex single. Whether it's worth doing is down to what you want to do next. If you're going straight on to do a multi IR then the extra twin time will give you more of a chance of a first-time pass. If you're stopping at the CPL for the moment then you'll be paying quite a bit extra for a twin and not achieving anything very useful (even though you would get a MEP rating you will need to take a proficiency check every 12 months to keep it current).