For the new Bayonets we're using overseas, I'm not sure about. But, as for the old C7 bayonets with the plastic sheath, I've got an answer why not to sharpen them.
One of the guys in my troop a few years back had sharpened his, not thinking that it was wrong. We had a change of command parade, with bayonets attached. When it came to the General Salute, and went to grab the rifle after the slap on the barrel, he slipped a little. He went to re-grab it, but grabbed the sharpened blade of his bayonet as it was falling, which of course cut his hand quite nicely. He let it go, but not very well, because the butt swung out and the blade swung in, and gave him a nice cut below his Adams' apple. It was a clusterf*%$ of ugliness on that parade, and he got dragged off to get stitches, and then dragged back to be charged for modifying his weapon.
Really nice, eh? Those old C& bayonets are used more for ceremony and/or prodding for mines, so I'd say it's safe to keep 'em dull.