Like all reflective surfaces the star diagonal is subject to atmospheric attack and dirt. The mirrors in an SCT are reasonably well protected by the corrector plate but there is still atmospheric exchange through the focuser hole, which eventually leads to problems - though it generally takes a long time (tens of years.)
As long as the diagonal is clean, free of dust and dirt, and does not look hazed or discoloured you shouldn't see any noticable difference in your images. A dirty, damaged or hazed diagonal will cause a slight increase in the diffusion of light around stars and objects (less light will reach a perfect focus.)
If you can get your hands on a new(er) diagonal then you can perform a quantative test by imaging a bright star at high magnification and analysing the diffraction ring patterns. Assuming a night of excellent seeing (or at least equal for the two exposures) then you should see a slight increase in brightness in the outer rings with the old diagonal. I doubt you'll see any noticable difference though!
If you're just paranoid, you can always get the diagonal re-coated, though you would want to get the original StarBright coatings to make it as good as a new diagonal.
Phil.
  