I'm a dedicated seat belt user, but I'm not crazy about the combination of soft top cars and three-point belts. If your car has a roll bar, then I think three-points would be OK (when I bought a new Miata a few years ago, I drove it home and parked it until I had installed the roll bar).
On the other hand, I agree with bighly: regular two-point lap belts seem like a good idea. I'm looking at this from the view of "before a potential crash".
The lap belts will keep you planted in the driver's seat, where you belong, even in fairly violent maneuvers. This way, you may have a shot at controlling the car (and possibly getting things back under control). All of this assumes that the driver does not panic, slam on the brakes, and close his eyes (but we know that AH drivers would *never* do that!).
As far as post-crash results, I realize that there are many people who are ejected from a crashed car and have lived. Despite some of these cases, it's my opinion that you're generally safer not to be ejected.
Odinary lap belts are cheap and easy to install. Here's some:
https://www.jcwhitney.com/webapp/wcs/stores/servlet/ProductDisplay?productId=4286&catalogId=10101&langId=-1&storeId=10101
As an aside, I've heard that some older cars entered in concourse events have had points deducted when seat belts were installed where none were originally fitted (and we may have even discussed this topic here once before). If it's true, it's a bad idea in my opinion.