There are quite a few FM attendings in my ED who have done EM fellowships. You do an EM fellowship b/c you want to do EM, and the hospital won't let you unless you do the fellowship. If you're in a larger hospital, odds are you have a trauma service, and if you're in a rural hospital, a lot of the trauma will be flown to a level I, you'll never see them.
It also depends on your residency and how able they were to prepare you as far as procedures, etc. If you have done a ton of intubations, lines, feel comfortable managing acutely ill patients, and documented your numbers, you may not need a fellowship to get a hospital/EM group to take you, especially if they really need the help.