a bayonet that is dull makes a more ragged and torn wound channel when thrust into a body causing more bleeding and some say causes more shock (how often have you cut yourself with a very sharp knife and not noticed it compared to cutting yourself with a dull knife), a sharp bayonet makes a nice clean wound channel that closes much more after up after you withdraw.
heard of this "it's illegal to sharpen bayonets in wartime"
as for what your local law is, that's a while other ball of wax.