Protection from wands/scrolls/etc, making low level spellcaster mooks a non-threat, as well as non-pure spellcasters (gish, rangers, paladins, etc.)
For example, a paladin's protection from evil won't be effective against an evil outsiders SR, but a cleric's will.
I doubt they wanted to make it super effective against pure casters, seeing as how it would make the game very unfun for the non-CO T3 'I like to blast' guys.
Personally, I like to abuse it on NPC spellcasters with SR (usually drow, etc), in order to manipulate effects that will affect my PCs but not itself. My PCs never bother to pick up SR, /shrug.
Such as per the Scattering Trap thread, set the CL of the spell just under the casters own SR