Inflatable PFDs don't count for USCG requirements unless you ARE WEARING it. So, having one aboard, you'd still need to have a Class III PFD on board for every person on board.
Also, wearing an inflatable on board is definitely easier to move around with, less bulky, etc. However, when in the water and inflated, it may be HARDER to swim, move around, etc.
As CB says, if it's cold out, wearing a foam vest style PFD will keep you warmer.
Depends on your main sailing grounds and conditions. I use a fanny pack style inflatable when I'm kayaking on the lake or other familiar, close type waters. But I don't intend to come out of the kayak, given that I have very strong rolling skills. When the wind swings to NW at our lake, it tends to be very gusty. I've been heeled enough to ship water over the cockpit gunwale. If the water is warm, I'll wear the inflatable (and put in drop boards) for those conditions. When racing, I think it's just easier to wear a vest style. When I'm out in the Force 5, the likelihood of swimming is high, so it's good to wear the vest.
I currently have a full size run of Kokatat Msfit PFDs from my kayak instruction days. I find them to be very comfortable fit-wise. However, they run a little bulky because they are higher waisted to accommodate kayak spray skirts (they have to have roughly 16 pounds of buoyancy no matter what, so the foam gets thicker.) I also decided to try a Kokatat SeeO2, which is a hybrid foam/inflatable. The idea is there's enough foam to be neutrally buoyant, maybe 7 pounds (I can swim under water with it on) but it also has inflatable bladders to give more than the 16 pounds of a Class III PFD. I thought it was going to be the best of both worlds - thinner and less bulky like a CE buoyancy aid, while still more buoyant than a USCG Class III. What I find is that it is about the same bulkiness due to the pockets to hold the bladders.
So, there are USCG Class III regs, and then there are European standards, the CE designation. CE has a "buoyancy aid" designation appropriate for inshore supported racing. Companies like Zhik, Musto, Spinlock and Gill make the CE approved buoyancy aids, and they are often used by racers. My friend who used to race Hobie 16's once told me that the Spinlock Zero was highly favored due to its low bulk design, but that sometimes other racers would protest because it's not USCG approved. I wouldn't have a problem using a CE buoyancy aid on a typical round the buoys race, because there's always a support boat if things go horribly wrong. Given I sail on a lake where I should be able to swim across when the water is warm, I wouldn't have a problem with a CE vest. The Fish and Boat Commission patrols wouldn't question if it was USCG, they'd just see a PFD-looking-thing on my person, and wouldn't think twice. If I got whacked, knocked out, and drowned while wearing one, well, I'd be perfectly ok with that. Sorry for friends and family who wouldn't, but then again, a USCG Class III won't roll you face up if you were face down and unconscious anyway.
In spring and fall, I would wear something. I follow Dr. Gordon Geisbrecht's 1-10-1 rule of cold water. Survive the 1st minute of hyper respiration and other cold shock, to survive the next 10 minutes of rapidly decreasing muscle control, to float around up to an hour before dying of hypothermia.
http://www.coldwaterbootcamp.com/pages/home.html
