Neat things the puzzle creator can do
What I'm talking about is not an XOR. XOR means either A or B, but not both. You could make an XOR with the OR circuit I mentioned earlier and a NOT gate attached to both of the objects being used for the or, and that's actually a pretty interesting idea to use for a puzzle, but it's different from the use of the NOT gate I'm suggesting here, which is an "A and NOT B" arrangement, where both A and B can actually be a set of more than one activating object. You're correct that you don't need a not gate if everything attached to the object is supposed to be off in order for it to function, but if you want two or more objects connected to one thing so that some of said objects are off and some are on in a specific combination to turn on the thing they're all connected to, then the not gate is useful.
In Boolean maths:
XOR means "(A or B) and not (A and B)"
what I'm talking about is "A and not B"
I see what you mean, though B should activate something for the player to want to put a cube on it.