Recently I have read comments that PVC flutes need to be swabbed out frequently in order to perform well. I think that is true, especially if the relative humidity is rather high. I like to keep the relative humidity in my music room at 60 % relative humidity during the winter months, and that means that I do have to swab out my flutes quite frequently as I am playing them.
However, this is not a defect of PVC flutes. I recently attended a performance of a well-known flautist at the Interlochen School of Music in northern Michigan. After every movement of the pieces that she was performing with her silver flute, I noticed that she pulled a silk cloth through the flute to remove the moisture. The fact is that moisture does build up in any flute with a bore that is non-pervious, and these little beads of moisture do affect how the air is able to pass through the flute.
It has been said that wooden flutes do not need to be swabbed out so frequently because the wooden bores of the flutes absorb much of the moisture. This, also, is true, but what this means is that wooden flutes need to be swabbed out after they are played, and I think that it is preferable that they be allowed to dry out fully before they are placed back in their bags or cases. For the same reason I leave my silver flute assembled and outside of its case, mainly so that the pads can dry out and to keep it from smelling funky from mold and mildew.