I prefer OWB holsters with retention (not on my IWB holsters, though). As others have said, it can help to prevent grabs, but it's mostly just for added security when moving around.

For this extra retention, I prefer thumbreaks to the "button/lever" style of retention both because it's what I'm used to, as well as the fact that thumbreaks do not have a mechanical component that can be jammed with dirt or debris. (The first time you get a rock stuck in your Serpa mechanism, rendering your weapon unable to be drawn, you might feel the same way as me. )

In the end, as with most things involving guns and holsters, it comes down to personal preference. I agree that extra retention may be a bit redundant for the majority of concealed carriers. If you're okay with an open-top holster, and that works for you, great.