Alright, I'm not a huge competitive battler (I've actually only battled other people maybe 3 times), so I can't speak from that perspective. Therefore, I'm hoping that you guys can explain why, for a Blissey, Natural Cure is the better option over Serene Grace. I have never seen a Blissey moveset other than my own that utilized Serene Grace. Blissey is one of, what, three Pokemon that can learn Aromatherapy? Why not use that instead? That way you don't have to switch out.
Maybe it's because I just prefer to use my Blissey as a PP-waster. Defense Curl until my defense is equal to roughly 400+, use some Thunder for 60% paralysis chance, Softboiled every 5-10 turns as needed, and Aromatherapy if they poison me. With every move at max PP, I can last for... 70-80 turns, just letting everyone I haven't killed with Thunder use their PP on me. This is why I use Serene Grace instead of Natural Cure. I'd have to switch out to use it, which would negate any Defense Curling I've done.