Most garages sell engine flushes and fuel treatments for 2 things. If it with forte products you are signed into the good garage scheme which is supposed to give customers a garage they can trust.
One is to be in the good garage scheme and two for profit.
The garage I work at pay £5 quid a bottle and sell them for £12 so a nice little earner.
My own opinion is that I wouldn't use it on my cars but I keep the oil changed between 6-8k in my daily and more frequently in my kit car when I get it on the road. In some times it can be bad for an engine specially if the oil hasn't been changed for a while just for an example the 1,6 hdi/tdci engines.
These engines are lets put it poorly designed and are up there with the worst diesel engines out there. If you flush one of these with very old oil in it the crap that has been kept in one place will have been disturbed. If this gets in to the filter for the banjo bolt in the oil feed to the turbo it can cause oil starvation to the turbo. Which will in not a lot of time run the bearing dry of oil and destroys the turbo. These engines are notorious for leaking injector seals which if not replaced will end up filling the engine oil with carbon/diesel. If the oil doesn't get replaced then it a full cycle and kills the engine.
Back to mx5 if you want to and have used it before it wont do much harm to it but it wont suddenly make the engine go bang as some make out more on diesels where it can be more problem than it's worth. If you don't want to and don't like them don't use them simple as that.