Oh no, it's certainly not just Merlin. Gullivers have also been adding accommodation, as have Crealy. The new Gullivers Resort they're building will feature a significant holiday village. And now John Broome's bought the Cornwall Crealy, they're also looking to add a significant holiday village. Wasn't Longleat talking about accommodation, and they're competing for a similar market to theme parks?
Some parks have done very well by adding accommodation. Walt Disney World was one of the first theme park resorts, and almost certainly wouldn't be nearly as big without all the accommodation. Europa Park's the biggest park in Europe after Disney. One of the factors behind their success was getting in on the accommodation market relatively early. A lot of enthusiasts seem to feel that Flamingo Land gets more visitors than it 'deserves', and the success of their Holiday Village may well have something to do with that.
Alton Towers is certainly the most rounded resort in the UK with a water park, spa, mini golf and high ropes course. If any UK justifies lots of accommodation then it has to be Alton Towers. But it's hard to see how the British parks can keep on growing their overnight offerings when the actual number of people visiting the parks is falling, and many of these parks seem to have falling standards. Given that Legoland is now the UK's most visited theme park, their new hotel probably will do well. A new Lego film coming out soon will also help their profile. But I can't see the bubble lasting much longer.
It's generally accepted that one of the main reasons that Disneyland Paris lost so much money, is because they severely overestimated how much accommodation they needed.