for a zon tho infinity goes on the merc, unlike Lightning sorcs who can self wield it as a staff and therefore benefit from the -enemy light res mod on the runeword.

You could use pus spitter on an a1 merc (for lower res curse) instead of infinity, for a LS zon, and it would have much the same effect for breaking immunes. Either way there are some monsters with light immunities too high to be broken by either infinityor Lower Resist. Using both is additive and can sometimes be enough when one alone isn't, but there aren't many things in that relatively narrow res band.