One small addition.
There is, as it happens, one thing that even those with milder cases of Mitral valve prolapse have to put up with:
Whenever they have any dental work done (even cleanings), they have to take antibiotics as a preventative measure against the (relatively tiny) chance of certain bacteria taking advantage of the prolapse and wreaking havoc they normally wouldn't be able to.
No, it isn't really a cross to bear or anything, but it is pretty much the only thing keeping those with the more "benign" cases of this disorder aware that they have it at all.
(Perhaps a better {simpler} way to think of what exactly is happening when the mitral valve "billows out" is that the valve flaps don't quite match in terms of size and fit together wrong, causing them each time they shut to snap back shut again wrong and buckle just a bit, allowing the leakage and making the characteristic "clicking" sound...)