Yes, being unusually cheerful and at ease with strangers offsets being cursed with cardiovascular and other problems because so many people struggle with social interactions. For someone to suffer in some ways but thrive in others is often a gift. We should learn to embrace the gifts we receive, even when they come at a cost.
While a cheerful, friendly demeanor is helpful in coping with any situation life throws at you, let's not forget the significant other problems they may have. Sufferers of Williams Syndrome typically have distinctive facial features that may catch stares in a public setting, narrowing of blood vessels that may cause shortness of breath, chest pain and heart failure, dental problems and increased anxiety and phobias.
I guess to an extent it is very nice that there is an upside to Williams Syndrome as well, but it is a big compromise to sacrifice things like a healthy cardiovascular system. Of course you would rather have an upside than no upside but I'm not sure it totally balances out.
While being cheerful and relaxed with strangers makes socialization easier, it is not worth the trade off to a person's health. Being physically unhealthy can cause great emotional distress that still affects a person even if they feel outwardly cheerful and upbeat. It would be better to have a cheerful attitude that comes from genuine contentment rather than a disorder.