I like manual, I learned how to shoot in manual and end up staying this way 99% of the time.
For me, the advantage of using M over A is ability to modify what I need and how I need faster - for example if I'm shooting indoors with crappy yellow lights all around me and I'm using flash, #1, I have to take WB into account (yes I shoot raw but I prefer to spend more time shooting and less time processing) #2, many cameras and my including, at A mode, in-doors with a flash attached will jump to 1/60sec. The only time I shoot around that speed is if I WANT MORE ambient light but when everything around me is yellow, and my flash is failrly colder, I prefer less ambient thus I shoot usually @ 1/125sec...
IF I were to shoot at Shutter priority, by default camera would open the lens wide open and thats is something that I don't like. I like to stay around 5.6-11 based on my environment therefore I HAVE to stay in M. NOW, my flash - thats is my main light. It gives me the juice and color that I need on faces and the scene.
Plus M, for me, provides consistency - b/c I know that at camera's 8 1/125, iso 400 at 12 feet my flash units will give me my exposure correctly REGARDLESS of how camera's builtin meter is set and REGARDLESS of my ambient light changes (I'm not referring to me walking outside midday to indoors with one little 45Watts light bulb).