I notice this with many of my pitchers, but especially with S. purpurea.
Though it seems that the worse smell from S. purpurea comes from two
things: 1) the constant water supply which stagnates, and 2) the fact
that the trapped beasties often turn out to be slugs, which add a special
brand of aroma as they decompose. Still I am sure that the smell would be
attractive to curious flies. I suppose one could test the hypothesis by
growing plants in separate areas and plugging the openings of one plant
with cotton and letting the other trap to a certain point. Then remove
the cotton and note fly attraction over the course of a week and see if
the pitchers with already accumulated and decomposing flies proved more
attractive to newcomers.
bob