How very strange... I've never heard this from a CKCS owner. It's rare to find a Cavalier owner with just one Cav and those with only one wish they could have more but can't only due to personal circumstances and not the breed.
As with any breed - registered, reputable breeders will do everything in their power to ensure their puppies do not develop such problems as MVD, SM/CM. The major problems come from inexperienced breeders, backyard breeders and puppy farmers.
Blackdogs - you do need to be aware of these key health issues and question the breeder in regards to these http://www.cavaliersa.com/health.htm. The other issues Arcane mentioned are rare and I wouldn't say are common to the breed.
My Cavalier, Lady, is 1 year and 8 months old. She has never had any medical issues. However, I do have pet health insurance as I know problems can arise over time (although, I would have this for any breed of dog).
She has been an absolute ray of sunshine in my life and I cannot recommend this breed enough to people who want a loving companion. Lady is friendly with strangers, children, and she never barks. She does suffer some separation anxiety as she wants to be around people all the time. Cavaliers are recommended to people who are at home most of the time (I work from home, so this suits me).
When I am in a position to get another dog, I will most definitely be getting another Cavalier.