Leaving the antenna tied down opens the spring coils and allows water in, also the plastic version develops cracks where water can get in.  When looking to buy one check for cracks and make sure the manual tuning knob on the bottom turns freely, if not pass on it.  

The MX-2799 tuning unit has the spring base mounted separate from the tuning unit allowing more flexibility of where you can mount the tuner and still have the antenna vertical and no chance of water entry from a tied down antenna.