Alton Towers might be in a league of its own, but it still needs to play by the rules and make sure it offers something for everyone. Flat rides like Towersart said are more than just another attraction, they are great tools for crowd control. Alton can't just keep adding coaster rides, it may work for Six Flags, but then, even they are struggling to make ends meet these days.
Alton Towers does need a few more flat rides, but they do need to be good. If done right, they will be far from a waste of money and space. They may not be the greatest thing at attracting new visitors, but people on park will want to ride them. Also, Alton has a good enough reputation to have a year or so without the need to have a major marketable coaster to keep visitors coming back, they get more repeat visits than most other attractions, and by adding a flat one year they certainly wouldn't loose visitors. They'd just make a great day out, even better.