Well alot of people like plastic fermenters because of the valve. I've never heard of anyone using it for bottling, I've only heard of it being used for a primary fermentor.
The valve is positioned right above where the lees settles so the thing is made for racking.
I havn't used one though, but I may one of these days. The pros and cons of a plastic fermentor are detailed pretty well in Shramm's book. Heres What I remember:
Pros:
-Easy clean up.
-The valve for racking.
-Often easier to add ingredients. No funnels.
-No plugged up carboys when adding fruit in primary.
Cons:
-Some meadmakers don't like the use of plastic, even food grade plastic.
-More headspace, but Shramm emphasises that this is not a problem. For one thing, the CO2 from the yeast pushes out any oxygen that might be in the headspace. This is why these are only used for Primary however, since headspace is more of an issue in secondary.