On the other hand I guess you're right about "people usually don't have that many unique sites open", so the original design value of 100 separate origins was probably purposely chosen to be on the large side, and thinking about it, I guess not having that many unique sites open usually fits my usage patterns, too. The unknown factor I can't really judge is how many iframes with potentially separate origins the pages I normally visit use, though.
Looking at it positively, one additional potential benefit could be that I have a few long-lived tabs that I always keep open – under the current model, this means that the content processes associated with those tabs never die and possibly slowly accumulate cruft and memory fragmentation from additional tabs that happen to be loaded in them (and later closed again).
Under the new model on the other hand, closing all tabs associated with a domain should be enough to get the associated content process to exit and free up really all memory used by those now closed tabs.
Can you explain this in more detail?
It really depends on what web sites you have open. If you have a single tab with an ad-laden news site, the overhead will be high, but if you have a bunch of Google Docs tabs open, there's no overhead.