Actually, empirical data gathered in Tracemonkey shows that specializing to integers where possible is a performance win even on x86.
This is especially the case in the common situation of code computing array indices. Since those end up being treated as integers in the end, if you have them as doubles for the arithmetic step you have to keep paying double-to-integer conversion costs. The same considerations apply for some DOM methods that take an integer, and similar considerations apply to cases when numbers need to be converted to strings (e.g. when setting .style.top on an element): this is a lot cheaper to do for integers than for doubles.
This is especially the case in the common situation of code computing array indices. Since those end up being treated as integers in the end, if you have them as doubles for the arithmetic step you have to keep paying double-to-integer conversion costs. The same considerations apply for some DOM methods that take an integer, and similar considerations apply to cases when numbers need to be converted to strings (e.g. when setting .style.top on an element): this is a lot cheaper to do for integers than for doubles.