I certainly see your point, Sam. I had a conversation with my parents about this a few weeks ago - and we all agreed that at the bottom line of it, from a guests' perspective the park is at fault. Or at the very least their day feels ruined by it. Even though it's out of Alton Towers' control, people who have travelled far and spent a lot on visiting, a power cut can have a real effect on a visit.
Most people on here don't get it I would believe mainly down to the fact most of the members on here have annual passes so a simple power cut is a non-issue. For your general guest that may only get one (or two if they are lucky) visit(s) a year, a power cut is a massive deal.
Although there's not much Alton Towers can physically do without spending tons of money on creating a stable power system to cover them during power cuts. It's a huge site and in the middle of the countryside. I highly doubt Merlin would see any benefit in investing such money either. Not unless power cuts begin to have a huge impact on the business yearly.