Hi,
This is indeed a good idea

The only available "protection-lenses" I know are tele converters, e.g. like the 1.4 or 2.0 ones, which extend your reach accordingly. However you will loose 1 stop with the 1.4 and 2 stops with the 2.0. Besides the image quality will normally degrade as well.
If you are using primes, you normally would like to get the best possible quality, so putting some pieces of glass, which degrade your picture quality is not that optimal. Nowadays high quality zooms like Canon´s L-lenses or Nikon´s G series are qualitywise really close to primes. Besides they are enviromental sealed. Maybe this is a more viable option ? - except these lenses are unfortunately real real expensive
No matter how careful you are sooner or later your carema will accumulate dusts. The most effective method for removing dust I know is using a blower to blow away the dust particles. For more stubborn stuffs, wet cleaning is my next option. Otherwise you can also let the your camera cleaned by professional services.
Recently some camera manufacturers have implemented dust shakers into their cameras, but according to certain reviews, the only real effective solution is Olympus' one... hmm.. I guess my blower will still have plenty to do
Cheers