Also consider that they are not covered by the factories or the dealer when they sit on their lots.
This is not something to consider regarding a decision to cover your TT or not. My cover protects my TT from dirt and tree derbies. Without it, it doesn't take long to get stains and streaks. If you install it correctly and cover the sharp edges, the cover lasts a long time. Mine is on it's 4th year. Follow the directions or look up a video on using it and it won't be a PIA. Takes me less than 30 minutes to cover/uncover.