Themeing doesnt add anything to a ride. The ride layout is what makes a ride good. As proven by six flags and cedar fair. They spend next to none on themeing but have amazing rides. Good themeing is very expensive, for example $100 million for expidition everest. Merlin dont have that sort of budget for SW8 so they would be better off spending the full £25million on track layout and a small amount on marketing.
I personnaly do not agree with your comment. Alton Towers is a theme park. Six Flags are amusement parks and as such dont really have any themeing.
For me I love the theme of a ride its what sets it apart from all the other coasters of the same type. A theme givea the ride a personality and offers more of an experience than just a tall coaster in car park, which there is nothing wrong with. Universal and Disney are masters at this and can spend £100million plus to achieve it and Disney's attention to detail is second to none.
Alton Towers themes are good but they don't have the money other parks do but that said they still achieve very good results for their budgets and very unique themes.