The Detroit Lions already had thirteen receivers on their roster. Northcutt is in the twilight of his career. Though he used to be a punt returner, he only returned a few punts for the Jaguars in 2008. This says to me that he is not as good as he once might have been, otherwise why wouldn't the Jags have still used him to return punts?
Trading Gerald Alexander away doesn't make any sense either. He has only played for two seasons. As a rookie he started most of the games and played admirably. In 2008 he was hampered by injuries. If he is healthy, it does not make sense why the Lions would trade a young safety, of which they only have five, for a fourteenth receiver.