Yes, but there's an inherent problem with that.
I assume by "taking the best advantage of a shadowpriest", you mean dropping a healer and counting on the shadow-priest's passive healing to carry the weight. To be even more precise, we're basically talking about divine star and using packed groups, correct? Because halo as a reliable passive healing mechanic doesn't work, and switching to a friendly target to cast the "bouncy heal" (forgot the name) is not a practical solution.
So right off the bat, even if you try to use different talents, your group's setup and strategy plays a very big role in whether or not the shadowpriest's passive healing can be relied on to fill the position of a dropped healer.
Furthermore, now you get into the old "intentional healing versus passive healing" argument, and the age-old question of whether or not passive healing even matters, or whether it just heals damage that would've been healed anyway. (Or in other words, overhealing or just plain old wasted healing.)
Personally I think the only time shadowpriests worked that way was back in Wrath, when our passive healing (through VE) could be stacked within groups to provide consistent, constant healing.
I'm not saying our current healing doesn't matter or help now - of course it does - but I don't think it eliminates the need for X amount of healers, because there's just simply too little control over who gets healed or when they get healed.
I digress that you may be talking about having shadowpriests switch back and forth between actively healing - ie renews, flash heals, shields, etc... - and dpsing. Or even trying to do both. Yes, that's possible, and I do that when I can. In limited situations I've even saved fights doing that. But I think Bliz has done a pretty decent job of nerfing our healing to the point that it's more of a "last resort" than something that we're utilized for. As basically every time a shadowpriest casts a pure healing spell, they cut off DPS by an equal amount, at that point you have to ask why the healers aren't doing that job.
There's emergency situations, sure, the but I don't think the game is designed in such a way that you need to do it that way. I don't even think we truly want the game to work that way. Designing the game around having people need to purposely pull dual roles within a fight would probably just discourage people from playing classes that can do it.
I feel "awesome" when I notice someone dying and heal them because I'm the only one who can. But if I consistently have to do this, I start to feel annoyed - is that person standing too far out of range, is a healer consistently dying, what's going on that I have to keep doing that?
"Falling from heaven is not as painful as surviving the impact."Revenant
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