That is a safer approach and one that chemists would feel happier with too. The paint manufacturers have the labs and the technology to see what works with what. As an example Farrow and Ball chemist said he was fine with Floetrol in his water borne paint. I believe Eicó are OK with Floetrol too, but absolutely not XIM Latex Xtender. Tikkurila we have been told not to use any additive other than water in the Feelings range.
It could be argued that they are just covering themselves being so restrictive. Fair enough, they make it and support it!
I also hear what you are saying @jrsl_interiors about never had an issue so far. Do we know definitively though that the life of a few jobs was not reduced by the fairy? Or extended?
It is impossible to prove either way, anecdotally. If the lab says xyz happens when you add a 3rd party additive, e.g. it affects say the resin, you can extrapolate that the film may break down prematurely. Us lot, looking, we can’t tell diddly really can we?
Until there is lab proof of no issues with other additives, I would say it is best to play according to the rule book that is supported, especially nowadays, with formulas changing monthly with UK brands especially. The UK made gloss paint in use even 4 years ago has nought to do with the gloss of today. I don’t know what the lab rats have done with new improved Fairy over the past 16 years, do you?