Tenga has a video showing how to use the Eggs on a dildo. Do a search for "Tenga Egg Video" on your favorite engine.
I used my Egg by stretching it to cover my entire penis once or twice, and I noticed a bit of thinning/stretching at the top. I found that using it only about halfway (covering ~4") both felt better (the texture is more noticeable) and makes the toy last longer.
My guess is that the main reason the Tengas are sold as single-use is that the Japanese seem to like the single-use masturbator concept. They'll work several times (depending on how far you stretch them), and the silicone makes them easy to clean and safe.