if the pipe is going the wrong way and
does not have fall to the drain it can collect
condensation and rain water from the vent
going out the roof... the water in the vent line can
basically close the vent off.....
if its pvc its no big deal, it will never rot away...
but if it still seems to work,
dont worry too much about it.
you could put a hood cjhimmney like cover on
your stack going out the roof to keep the water from collecting
in the vent line.