I'm wearing a PFD/buoyancy aid almost all the time while windsurfing, be it in ultra light wind on the Serenity or blasting at high speed in strong winds.
The question of wearing or not a PFD is indeed a risk analysis between life and death in case of a mishappening. As simple as that.
It does not matters whether you are a novice or an experienced rider simply because shit happens at any time without prior warning. Gear can fail and break without warning while out at sea (mast, boom, fin, sail, mast extension, universal joint, bolts, etc...). A wipe-out while blasting at high speed can hurt you seriously (broken ribs or shoulder, muscle bruise).
3 advantages of wearing a PFD :
1) it helps you with more buoyancy while waterstarting resulting in less fatigue and less energy loss,
2) it keeps your upper body and core organs warmer, even worn over a lycra top or a neoprene shorty and,
3) it protect your upper body and vital organs against impacts during wipe-out (boom, mast, board, fin).
Key is to choose the correct PFD best suited for windsurfing. I've tried many different brand and type but the best PFD are those made for jetski or waterski. They are specifically made for high speed water activity and therefore are snuggly cut to your body. They have multiple heavy-duty straps/buckles, and are made with tough and resistant materials. They are designed with a long and low back, covering your waist and protecting your low back/kidneys from bruising.
The choice of harness is key because of the low back cut of these jetski/waterski PFD. Wearing such a PFD with a racing seat harness is no problem, providing the harness does not have a high back support, which can interfere and superimpose itself with the low back support of the PFD.
A waist harness is kind of problematic with such jetski/waterski PFD. The best trick is to wear the harness directly over your body and superimpose the PFD over the harness. Let the harness hook protrude out of the 2 unziped sides of the PFD inbetween a pair of straps/buckles. I have seen some PFD specifically designed for kitesurfing that have such a "hole feature at the level of your navel" that allow the underlaying harness hook to be freely accessible throughout the overlaying PFD.
Some PFD designed for windsurfing are perfectly compatible with waist harness because they are cut with a very short back/front. However, they are bulkier as compared to the jetski/waterski PFD and tend to move up your chest over time. Tying or strapping the PFD to the harness bar is a good way to prevent this kind of PFD to move up your chest.
Whatever PFD you choose, I don't recommend the very bulky standard sailing yellow life jacket with a high collar while wearing a harness. These life jacket are OK for kids or novice learning the basic of windsurfing, but once your skills improve, you will sooner or later want more freedom of movement, ease of use and comfort while wearing a seat or waist harness. Sure, the standard sailing yellow life jacket is the one that does offer the best buoyancy and best survival chance because they will in effect save your life if you are knocked out unconcious - because the high collar support your head out of water and prevents you from drowning-, but a snugglier, thinner and softer PFD that follows your body movement is much better IMHO than wearing the bulky sailing life jacket or, worse, wearing no PFD at all.
Safety has no price, especially when it come to your own life. Wearing a PFD is the first step you take to increase your own chance of survival without depending on external factor that you cannot keep under control 100% of the time (wind gusting, gear failure, distance from shore, swimming ability, search & rescue crafts, etc...). Choice is yours...