Most swordtails for sale are already sword/platy crosses. The swordtail's got most of their different colour sequences from being cross-bred with platys. So, they are not lame, they can reproduce.
Also, while male swords are the only ones that have swords, females are able to spontaneously change sex in a single-sex environment (like in Jurassic Park, but with fish instead of dinosaurs). You may see the female begin to become less round and the bottom of the tail extend into a sword.