I wonder why they never been mentioned in the endless discussions about evolution, or did I simply miss it?

The transposons changes the image of chance and evolution as the typing monkey who produces an endless row of random letters into a picture where chance mixes whole pieces and trunks of instructions instead of letters.
Transposons - The Jumping Genes

Retrovirus and Transposons

transposons are a great piece of genetic regulation. but not the main factor of evolution. the main fact is mutation. since mutation activates Transposons or disables, which will lead to a diferent phenotype

That's not quite how I understood the thing. As far as I understood the thing, the transposons already cause mutations, and they do it in a faster way than other things.

well yes they cause mutation when they move in between a gene, disable it.