When I first started paddling, the accepted wisdom was back rests were a "Bad Thing" in that they restricted your ability to rotate your torso. This makes "proper" paddling technique and rolling generally benefits from the ability to lean back. Most backrests available in commercially available boats are have these bad characteristics, but I think a backrest is essential for comfortable paddling. There are backrests that let you do everything you want while still providing support.

> At the Paddlesports Expo I sat through a class about basic technique.
> The instructor said that you should rotate your body. I found that
> this seemed to make the backrest on the seat utterly useless. I know
> I'll hear about properly adjusted boat, etc, but I tried alot of boats,
> and found this the case. Whenever I leaned against the seatback I
> would use my arms to paddle, not my torso.