I don't deny that in some cases. My parents, without fail, moan every time we ride about how the Wicker Man pre-show is dull and unneeded, and say that they should turn it off. In fairness, I will contradict myself and say that Alton in its modern form might not necessarily be the type of park where theming is of paramount importance to those who visit.
However, I do think people appreciate theming and story-building if it's there. If Brits didn't like immersion, why would they go to the Florida parks, which are full of immersion and narrative, in such large numbers? And why would Merlin even try and theme things if the public didn't care about theming? Theming on rides doesn't exist solely to please enthusiasts; if Merlin didn't think it was worthwhile for the business to add heavy theming to rides like Wicker Man, and they didn't think that the wider populace would "get it", they would definitely save the money and leave it themeless. Theming costs a lot of money.