I never thought starting a sentence with "but" or "and" was against the rules. I've always considered it a matter of style. I see it most used and overused in green writers who have not yet developed style. I do not see it used in the published books that I read.
Just because something is grammatically legal doesn't mean it should be used willy-nilly. On occasion, for emphasis, one can get away with it, but I think it's a matter of getting away with it rather than it being super effective. I cannot imagine anyone ever thinking "Oh, look at the masterful way that the writer has started that sentence with a conjunction!"
Personally, I do not begin sentences with conjunctions outside of dialog.
Just because something is grammatically legal doesn't mean it should be used willy-nilly. On occasion, for emphasis, one can get away with it, but I think it's a matter of getting away with it rather than it being super effective. I cannot imagine anyone ever thinking "Oh, look at the masterful way that the writer has started that sentence with a conjunction!"
Personally, I do not begin sentences with conjunctions outside of dialog.