MC's new [caption] field is saved to the files' IPTC tag header.
This means you can set captions to your files and send them to friends and family with a very high probability that they'll also get to read the caption you set.
Other applications will also be able to read the caption if they have that capability, which could help with such things as creating a DVD slideshow or web album.
If someone else sends you an image with an IPTC caption present, MC will import it to its own [caption] field.
The [caption] field should not be confused with the "Caption" option found in the image playback settings. They are two seperate things entirely, and should remain that way. I also use quite a hefty expression in the playback caption option, and the [caption] field features in that expression.
I guess a lot would depend on how you use the caption field.
Personally, I tend to use it mostly for adding a potentially witty punchline to a photo, which I find adds a little something for the audience when played back later on the TV.
my slideshow caption expression currently starts off with:
if(isempty([caption]),if(isempty([events]),if(isempty([places]),[subject],[places]),[events]),[caption])