Wow, this thread came back with a lot of action. Lol. Okay...
I don't know, I've never had a problem doing as pictured below. Unless someone is really going out of their way to look at your cards, other players won't be able to see them. Even in the photo I wasn't able to see the cards through the camera, but I can move my head to a position where I can see them both clearly. And I keep the stub relatively far away from my hole cards so that everyone can see a clear separation:
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I looked this up and was surprised to see that this is an actual procedure according to a WSOP dealer's guide from 10 years ago. I personally don't do this and was taught to always mix the stub with the muck after the river. Literally mix into it, not just place on top. This also helps prevent people from asking to see what the next cards were. The WSOP guide says this is to retrieve the top the stub in case of an irregularity. Okay fair enough. I have never personally had such an irregularity that required retrieval of the stub, but I suppose the possibility exists and maybe that outweighs any other existing pros of the procedure I follow. I have never dealt for the WSOP, but I have dealt the WPT and never heard there were any issues with how I dealt.
No, that doesn't work in my opinion. Some others have commented why this might be troublesome, but one reason I see is that sometimes the dealer reveals a board card before action has completed. In those instances, it may be necessary to use the river card as the turn and then reshuffle the original turn into the stub and deal the top card as the new river. I suppose if the stub is separate from the muck (which I said above is something I don't do), then maybe this could still work, but it doesn't solve the other complications that others have already mentioned, like two face down cards just sitting there on the table in a very vulnerable position.
Also, nobody does stub counts here? lol That's generally done after the river has been dealt and I don't see it as a practical procedure in the first couple of rounds when there's more stuff for the dealer to do as opposed to at the end.
Sure, it seems simple enough to just set down a stub and then pick it up when you need to. But I also always tell people that poker is the easiest game to deal... provided that you can deal perfectly every time and neither you nor the players make any mistakes. Once you factor in mistakes and irregularities, it easily becomes the most difficult game to deal in the casino. The same with dropping the stub. If you can be sure that you never have a mishap with the stub that's not in your hand... the deck stays square and nobody ever touches it, then sure there's no problem. But the reality is that once you no longer have control of the stub... well that's just it: you have no control of the stub. Anything can happen to it, and eventually something will. As
@glynn already mentioned, dropping the stub introduces unnecessary risk and slows the game down.