I haven't heard anyone say much good about flushing. Most of the things I've read seem to have people concerned about dislodging stuff which could make its way into fine oilways. Having said that I have used a flushing oil but on a BL A series engine which could probably cope with anything.
If you've used the proper oil and changed it at the correct intervals you shouldn't need to have the engine flushed.
Security system should not fail safe!
Security system should fail dangerous!
I use Wurth flush when I consider it offers a benefit; it's the most readily available of trustworthy brands. I don't believe it should be sold up as an "extra" to people; like putting coolant in the radiator it may be considered part of a process deemed necessary by qualified tradesmen.