Originally Posted by
gcottay
It matters for the same reason a smart person would seek out a professional mason on a brickwork question. A professional wrench often has more experience than the average home mechanic.
The only real objection to them is that they're ugly or uncool or will give you cooties. Their mass doesn't matter, nor does any aerodynamic inpact to anyone who isn't racing, and they can stop an overshifted derailleur, so they do have a practical use.
It's highly unprofessional to work on something and fail to put all the parts back on it, unless you're instructed to remove it.