Covers of any kind hide stuff that should be cleaned and maintained. I have seen several machines that look like showroom (unused) condition with serious amounts of corrosion underneath the covers. Hell, the fact that many of these machines are in unheated shops and suffer condensation should be reason enough to leave them exposed to atmosphere and any air movement they can get.
Nothing wrong with using diverters and shields during operation but leaving a plastic/rubber "blanket" like covering over any part of a large iron/steel machine is folly.