The trap "especially on a negative pressure AH" depends on the siphon effect to drain, you lose that "some" of that effect with the open T. If you close the T and it will draw/siphon the water all the way to the main drain, you end up having more volume of water drawing the water through the trap. Personally, I never leave any T open for critters to crawl into, a small mouse could get into a 3/4" opening, maybe even a large mouse could.
Originally Posted by vstech
Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law
"Never let success go to your head, and never let failure go to your heart". - Unknown