non "native" iso's

[mjs1973]

I read a blog post by a pro once that said by using the Lo ISO settings, you are going to introduce more noise into the image. The same way that you do by using a high ISO setting. Not sure of the technical reasons why... He went on to say that it would be better to us a neutral density filter to lower your shutter speed than to use the Lo ISO setting. I have never tested it myself so I don't know if this is true or not.

[Loupey]

On the newer Canon bodies I really can't see a difference in image quality using ISO 50 or 100 verses ISO 200 or even sometimes 400 (if there is little shadow in the scene).

I don't think there are "downfalls" to using Lo. It's just more tools to offer you more control. If you need longer exposure times or want a shallow DOF in bright light (like a portrait outdoors), then having Lo is extremely handy and preferred over using ND filters IMO.