I can see both your points but the machine does have the advantage of being able to reverse its flow which you can do with mains but it can be a bit of a pig (cutting pipes etc) whereas the machine can go straight on the pump valves.
I used to for years mains flush after leaving some chems in and allways found it ok, it did the job, the firm got a machine purely because the manufacturers were starting to insist on it so they spent 680 plus VAT on a kamco (seven years later they tight gits have brought a new one
) and put me on the powerflushing, which I found mind numbingly boring, and found that it wasnt that much better than a chem/mains flush (taking the rads off the wall etc etc) but it was a bit easier on labour.
I must say I like the machine and it does seem to do the job well (maybe not a panacea as some lads think but ok?)
But does anyone know if PART L and the manufactures are insisting on it? ( i would look but i am waaay too fat and lazy)
I think it comes down to personal choice at the end of the day, and speaking personally I would not be happy whacking a new system/combi boiler on an old system (i would prefer replacing all the old rads and pipework but the client usually dont want to pay for it and we aint doing it for free
) without powerflushing it, I also like popping in x400 a few weeks before as well as I am a paranoid b*****d