It can be a very effective technique, though I'm certainly no expert. I do find two problems. I guess it's just practice: a) it's slightly slower moving from the pickup (when plucking the E string) to get your thumb up and over and not just anchoring on the A string. b) I find that anchoring allows me to leverage the plucking fingers, a bit like the way the thumb is built to grab things. That's not to say I'm grabbing or digging in when I pluck, but that as an anchor it allows me to havesomething to "pull against". When floating, that isn't the case. It feels slightly less stable.