That's because both of the Merlin ones where shafted by ground-work issues.
If Disney pay more for precise track fabrication then I hope they got their money back. And you kind of made the point yourself really alistair, Disney are what make the rides amazing, they have done similar with MS and Intamin too. So I fail to see how choosing a gerst reflects on the park. They are very competent manufacturers, I would hazard a guess that the only reason Disney and others haven't used them is their coasters are not high throughput.
(Tells self off for going off topic)