In most countries that have parlimentary systems, it is rare to have prime ministers that serve 8 years. Also, the parlimentary system itself is extraordinarily unstable with new goverments and fringe groups constantly able to cause upheaval. That is one of the pluses of the U.S. system of government is that it governs much more consistently that any other model I have seen world wide.
It's certainly an advantage to have a more consistent model for the US since the global implications of any inconsistency or change in the governance of the US would have much more of a far reaching impact than that of the UK or Australia, as examples.