If I recall correctly (and I'm not 100% sure on this), don't you get 5 hours off the IR if you've got a CPL? Not quite as much as the 10 hours you get if you do it the other way round, admitedly, but...
However, these are minimums. How many people complete the course in the minimum time? So, really, any reduction in minimum hours doesn't necessarilly equate to a reduction in actual hours flown, except for a very few naturally-gifted people.
I suspect the reason many people do the CPL first is because of the options it opens, for example in aerial photography or flight instructing, while saving money for the IR. If you do the IR first, you can't get a flying job until you've got the CPL. Of course, if you've already got the cash for both, and you're doing them back-to-back, this point is not relevant.
FFF
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