The short answer is, no. You don't NEED to do a fellowship when you're done. I haven't seen the numbers which you quote (BTW, what about the other 13% of people?), but it's not surprising. I think most people who train at large academic institutions will do fellowships.
Basically, the reasons to do fellowships are highly variable. At Case, there has not been a resident go straight into practice w/o fellowship in like 15 years! There's a huge "internal" push to do fellowship here because we're a large academic institution that prides itself on producing academic physicians (BTW, all of our attendings did fellowships). So if you want to get a hard core academic job, are on the chairman track, or want to do big time research, it probably is important to do a fellowship. Other people do fellowships to become more proficient at a particular field of interest or fill a deficiency for something they didn't pick up in residency (i.e. a sports fellowship to become proficient in shoulder arthroscopy). Then you have people like me who want to to COMPLEX stuff in a defined subspecialty (i.e. complex spinal deformity cases, multiligamentous sports injuries, pelvic/acetabulum reconstruction, or revision joints). Furthermore some issues in orthopaedics, i.e. Tumor, peds, and hand probably shoudn't be undertaken in practice w/o a fellowship in the respective field. For instance, a generalist can have a sports, joints, or even trauma heavy general practice w/o having done a fellowship in any of those fields. With that said, I plan on doing a sports fellowship because I love sports and want to be "the man" someday for some college or professional team. Will I do some joints, yes. Will I do some trauma, yes. Will I do some spine, no
. . . . .but training at case I could if I wanted to.
Also I look at it like this, an additional year of training couldn't hurt, right? it probably will make you more marketable no matter what type of practice you do, community, academic, or somewhere in between. And it's probably better to do fresh out of residency when you're used to living like a resident. After all, you never know where you're gonna end up down the road and it would be hard to go back after living the attending life. Hope this helps. It's just MY opinion.