Which does make me wonder why the heck the Ancients designed the Stargate to disassemble stuff that enters the puddle in a situation where it's got nowhere to send it.

What situation are you thinking of? The gate can't open at all if the exit can't open. If it can open, there's the big swoosh that blows several meters ahead and disintegrates anything in the way. If you mean the iris, it's set something like a few microns ahead of the surface, so atoms can pass, they just get smashed on the iris. It's not like the gate is going to constantly measure the size of shit that goes through and visually check if someone didn't just put up a wall in front. Also, the Ancients themselves did use shield technology that exact same way, see the Atlantis version, so it is all on purpose.