Given how the plays were reconstructed after his death for publication, and given the variation in phrasing and spelling between typesetters, I'm not willing to worry too much about whether the words in my Folger's Library edition are the exact, literal words of William Shakespeare (or however that's spelled), because they're probably not. But that doesn't mean I'm going to grant co-author credit to Marlowe or Middleton or whoever, because that is tedious hair-splitting.

As an academic exercise, this is interesting at best, but what is with everyone scrambling to discredit Shakespeare?

I despise "Shakespeare didn't write it" conspiracy theories. They always seem to be rooted in the idea that some random commoner can't be a literary genius.

Usually they attribute some ghostwriting nobleman. At least Middleton is a peer of Shakespeare. Makes this easier to swallow (and more believable).

Yeah, I guess collaborations are possible. Okay. But, like every single writer in history who has ever read someone else's work and liked it, Shakespeare was also probably heavily influenced by what he read, and especially whatever he'd read most recently. Certainly more parsimonious that way.

The only Shakespeare play that was almost certainly begun by Shakespeare and finished by someone else (at least as far as I'm concerned) is "Troilus and Cressida." Why? Because there's a huge, jarring gearshift somewhere in the middle. I don't remember whether the break is after Act 2 or 3, but basically all the plot developments up to that point are scrapped and a whole different plot is set in motion. And no, it isn't one of those "OMG PLOT TWIST" kind of shifts - more like (1) Shakespeare put away the play, came back to it months later, and couldn't remember where he was planning to go with the plot, or (2) Someone else picked it up and finished it without consulting with W.S. I tend to go with theory 2, because the second half of the play really sucks compared to the first.

"All's Well" is one of WS's lesser plays. So claims of co-authorship on this one (besides the fact the plot was taken from Boccacio) brings up questions as to the motive of the theorist.

Almost nothing really good is ever produced by one person alone.