I recently went to using a splash top or semi dry top for all of my wilderness paddling, and day trips especially, instead of a rain jacket. The distinction to make is that a dry top is usually gussetted with latex or similar and designed to keep you almost completely dry. I dislike being closed off in a hood even when its raining, so a waterproof hat and a splash top is prefect. I use a kokatat gore-tex model and my brother uses a supplex one made by NRS. They pack light and are a good wind layer when it's cook, and most of them are breathable and comfortable to paddle in.
Is it necessary? probably not since it's basically a rain coat in my opinion, but if weight and space saving is a concern, they sure can be nice to have. I do most of my paddling in MN, and I like the splash tops. If you decide you want one, you don't have to splurge, I got a nice one because it was on clearance, but it's basically a back up to a good rain jacket that is comfortable to paddle in and doesnt have many (or any pockets) in most cases.
-Thats my .02
Erik