Alright so I figured it was better to show than tell. Here's some good good afflocks that I follow. The first video is a kara clear it shows how amazing affliction aoe is. The second is a guide for maw. And the third is a +16 Nelths. You may be using the incorrect spec or rotation if you're suffering from ramp up. If everything is dying fast you should be using sow the seeds. For higher level stuff it won't die fast so you won't have that problem. That's not to say that there is no issues with ramp time. But they can be worked around. In my experience demo is better single target but doesn't keep up with multi target or aoe.
Geletyven - https://www.youtube.com/watch?v=LThZyau2CBA
Yipz - https://www.youtube.com/watch?v=yzs4fCeYAq8
Terryn - https://www.youtube.com/watch?v=lxaLGdXxWI8