Seems like something that you build into a macro, at best.
Also, doesn't Rune Growth trigger global cooldown? If it does, then you have to ask the question "Is it worth it?" .. For normal mode, you'd have to test with and without rune growth in the macro. See which is higher DPS: Keeping Rune Growth up full time vs. just getting a few more attack skills. For shield form, if you can hold the agro while losing 1 attack skill every 15 sec, then sure use Rune Growth.
If Rune Growth doesn't trigger GCD, then holy c***! Put that in every macro!
Edit: To clarify, the macro I have in mind looks something like "if Chain Drive buff, cast Rune Pulse; elseif not Rune Growth buff, cast Rune Growth; else cast Shadowstab;" very pseudo-code, sorry. One of these days I'll put in the time to write the real thing.