less weight (no marked increase in windage as the halyards are right next to the stick)
Easier on the hands.
no "fishhooks" to catch on a sail
the quality of line
has improoved so much in the past 10 years that all rope halyards are becomeing the "norm" rather than the exception.
Easier to replace.
Less expensive (unless you're going for the racing spectra cored halyards)
My friend replaced all his halyards (main,genny,spin) on his 1983 C&C 29 last year with 3/8 in. sta set x