Say I have almost no RP and the only two runes I have left are and unholy and a frost I'm going to Oblit because it can keep my rotation running smoother and still deal the same damage. How does this make it run smoother? Well 1 Rune = 12 RP while 2 Runes = 24 RP. 20 RP = 1 FS. 1 FS = 45% chance to revive a depleted rune. Same situation, You're out of RP and only have 1 Unholy and 1 Frost left. What do you do? SR and get left with an Unholy and the inability to FS and possibly keep your rotation running.
Maybe you could do a combination like an SR and a plague strike but that just seems like a waste of an Unholy rune for very little dps gain.
Say you have only one Death or one Frost rune left and nothing to match it with for an Oblit, that's when I SR.
Say I have full Runes (two of each) and about a half a bar of RP. I'd use SR when up until my rotation slows down; if it even does. Why would I use SR then? Because I have plenty of resources to keep my rotation going. I have a large amount of FS still in my rotation to revive that one rune I just spent.
To me it's all about what keeps my rotation going.