The -theory- of it is basically that the spell doesn't create a magical burst of energy, like evocation does. All of the magic is used to create a quantity of nonmagical energy, which then is no more subject to SR than, say, a flask of acid, or alchemist's fire.
This works okay for acid, I think...and maybe certain cold/ice effects. Those things can be said to have physical existences that can be called into being and that no longer require magic to sustain. Lightning is a bit iffier, I'd say. What exactly are you conjuring? Where is the lightning coming from? Fire is by far the worst, since a fire conjuration ought to simply result in a puff of flame that quickly dissipates. What holds an Orb of Flame together? What makes it fly straight and true?
Yeah yeah, I know. It's magic. But that's the problem. If it was magic, it'd be subject to SR. The whole idea is that it's NOT magic. But then it goes ahead and behaves -exactly like evocations-. I don't think you should be able to have your cake and eat it too, frankly. I'd allow acid orbs as written, since it's in line with Acid Arrow and Acid Cloud...and not too terribly hard to concieve of. I might also allow spells that pelt people with rocks, or hurl sharp ice spikes...that KIND of thing...to be SR immune. It makes a certain degree of sense.
But I took some issue with Conjuration muscling in on Evocation's turf. A real energy blasty thrower ought to be an Evoker. That means YOU, Orb of Force!
Thank you.