transposons

well iam reading this paper for some evolution class, and its about two drosophila youngsters with similar sequences for Adh gene....that bit donesnt really matter all i want to know is whats going on with these transposons....it seem that finally a transposed sequences has developed a function and not just messed everything up....this was decided upon by having a increased number of slient mutations, and no new stop codons....thus positive selecting for this sequence....well anyway why is this so uncommon to have transposition with function, i understand that it would also be a new function by stealing parts of an orginal sequence.... or at least the first part, because it retains the stop sequence. and if this retrotransposition works out and gets function, then really whats the point of introns.....stupid introns