From "Take Two: Reusing Homebrew Yeast", page 58, BYO Sept '13
"Some homebrewers will reuse the yeast cake in its entirety, transferring a fresh batch of wort directly on top, into the old fermenter. This is a terrible idea and is contrary to everything we know about proper sanitation procedures. They will argue that they have never had an issue with this method. My response: If you haven't had an issue yet, it's just a matter of time."
Anyone else read the article and find the above a little odd? I'd like to know why this is such a bad idea but can find no direct answers upon rereading the article.
I understand there are some good reasons to choose not to rack wort onto the cake of a just-finished fermenter. Such reasons tend to mirror those pertaining to reusing yeast in any circumstance--if it has undergone excess stress, is mixed with too much trub or hops, is too old, is infected or suspected of, etc.
As for sanitation issues inherent in the reuse of a fermenter and cake, I fear I'm at a loss.