And due to the reasons John stated some engine plane combos will run better then others without the pressure line. I have had planes that didn't know if the line was attached or not but the same engine in a different plane would hardly run at all if I forgot to put the pressure line onto the muffler nipple. For the most part they tend to run poorly without the pressure line.
I lost my favorite Kaos when the muffler header broke on the down line from a stall turn, I hit the throttle and all the engine did was sort of burble and wouldn't rev up for the pull out.