I don't get it either... I'm perfectly happy with my 1/2 carat solitaire - brilliant cut, good color and clarity, and from Tiffany & Co
Mr. aephi alum was a poor starving grad student at the time, and couldn't afford the Rock of Gibraltar - nor would I have wanted it - a big rock would look silly on me.
When I first got the ring, a few people told me "oh, when you have more money, you can get a big rock, and use your current diamond as one of the side stones."
If I did that, it wouldn't really be my engagement ring any more...