You can read a lot of science, some of it remarkably well informed (IMO) on the various forums regarding this. Olfaction and gustation etc etc etc, backed up with scientific papers.
And then you read Hutchinson's writings where he promotes flavours quite strongly (no pun intended
) and attests to their attraction properties.
Fish, I am informed from these bait buffs,cannot detect oils yet Pukka Salmon was oil based and both McDonald and Springate witnessed fish swimming across their baited spots and then diving down to take the baits. On a day no-one else was catching so work that one out!!!! Was it the flavour or the oil that attracted them? You might assume the oil as it would have risen through the waters layers and been noticed by the carp cruising in the upper layers, was the oil carrying the flavour? Salmon oil would act as both surely?
I used little bags of boilies and salmon oil to good effect before but whether the oil was attractive or the boilies I couldn't say. Scopex and chocolate malt from Hutchie,Rosehip from Mistral and Richworth Tutti's are all winners proven time and time again. Mistral and Richworth are basically 'carrier' baits I believe so is it the carbs or the flavour that attract, or is it both.
In essence, (sorry), I work on the terms that if it gets results then don't mess with the formula regardless of what the science would have you believe. I think they do no harm and in a day and age when flavours are frowned upon by many it might just be an edge to use them.