I have no problem with an artist signing a piece of art...as long as it's outside of the original art area i.e. somewhere in the border area. The art is not ruined in any way and if future caretakers don't like the signature then they can either not look at it or matt the art.
I had Stan Lee sign a few pages of vintage Marvel art decades ago but I had him do it in the margins. I don't consider that "defiling" in the least - now had it been in a panel, well that's a whole different kettle of fish. In fact, with VERY FEW EXCEPTIONS (Ditko, Frazetta, Sprang...), I would most likely not buy the page just due to the signature placement in a panel.
People allow creators to defile comics all of the time by getting creators to place obtrusive signatures all over the cover . Let's not have that stupidity creep into the original art field.