Fred's argument is simple: when you upload images to MLS sites and they get syndicated to national and regional sites the images get mangled. There is no standard for aspect ratio or anything else so you will likely be surprised to see what your images look like after they get uploaded, downsized and passed around site to site. If you provide an agent with a slideshow or some kind of tour you have more control over what the images look like and you can insure the images are higher quality and have more impact.
Yep, I agree with Fred. The thing I would add to Fred's argument is the size aspect. With images, size matters! On MLSs and sites that listings get syndicated to the images are typically smaller than you'd like. Let's face it, big images have more impact than small ones. So a high quality large image get's more attention than a small dinky one. This image size issue is why the full screen mode in both slide shows and video is so important. There's nothing I like better on a tour or video than to have a full screen HD mode that I can click that will fill up my 16:9 27" iMac screen. This is the ultimate test of a tour or video. How does it look at 27". I think you should make sure your tours and video looks good BIG! Some tour sites appeared to still be concerned with bandwidth. I say go BIG or go home!