Just curious, do you have any good examples of very large scale systems that need system-wide transactions? I might miss something obvious (I'm not actually in the database area), but I couldn't come up with a good example.
I'd suspect that if those don't qualify as 'very large scale' to your definition (which would be entirely justifiable) that such systems don't exist because they're not currently possible. For example I don't think it's possible to implement Facebook with SERIALIZABLE no matter how much money you throw at the problem.
(I'd love to be proven wrong of course)
Normally, whenever physical objects or money is involved, transactions should be considered.