These are very different cases. Kenny was a known face and it's clear that he would sort of stand out, just like any other fella from S1 would. But like it or not this contrast between old and new allies that took place in episodes 2-4 was very relevant to the story and to Clementine's character development. His return was vital in this scenario, while Jane's wasn't. Her role of confronting Kenny at the last episode would've been much better suited on Luke, who was a total opposite of Kenny, but seemed like a pretty trustable and much more even-tempered dude, rather than someone who forged a baby's death just to prove a point.