Originally Posted by
Selesnya
You gain no mana from utilizing rapture during the LMG proc. You would have gained the exact same mana without the LMG proc. Rapture is tied to its ICD and just that. Whether you have the LMG or not is irrelevant, unless you're trying to play the rapture cooldown by PW:S spam, hoping to shorten the gap between procs, but that's not what you're implying. Even if you did that, I highly doubt you'd be able to track your active PW:S on the raid and predict the time of your rapture procs as to not waste mana on too early or too late PW:S casts.
The LMG allows you to conserve mana and to temporarily increase throughput, which is a lot different from "gaining mana". The LMG has an opportunity cost, which usually at least is atonement. If you're spending each and every LMG proc on PW:S spam, the opportunity cost of your proc is 27000-40000 mana, the cost of 2-3 PW:S. Which in turn again means that you're wasting ~25000 mana each time you don't spam PW:S but keep smiting while the LMG procs. But that is very theoretical, since you can't guarantee that the PW:S you cast actually get absorbed. The fact that you're able to spam 40000 mana during a proc doesn't mean that you "gained" mana. Other than that, what you said about halo applies to rapture as well.
We can happily debate on how much mana the LMG saves with what playstyles. All I want to say is that relating the LMG proc and rapture to each other is incorrect.