It hardens them and will reduce their life considerably. We are delighted with it and play it all day. Lots of moisture condenses and starts to run out the end. Some of the moisture gets through the oil and swells the inside of the flute. The wood on the outside is dry.
Phenomenal forces build up and the wood cracks on the outside.
Supposing we had played it for 10 minutes, then dried it and put it back in its case. Not too much moisture would have got past the oil, but that which did will now start to diffuse throughout the instrument. In a few hours, we take it out, play for 10 minutes, dry and put away. Again some moisture gets through, but has time to diffuse before the pressures get too high. This is the principle behind breaking in a woodwind instrument.