If we look at how any group in America has achieved success, it has been though nationalistic priciples. I don't want to speak niversally, but it seems that in the U.S., the answer it's very desireable, if not necessary.
As to the mental health of individuals, that's a harder question. I would probably say it's not necessary. Strong ties to ones racial/ethnic/national/religious group is one way to construct a positive identity of oneself, but not the only way. We may not look at a Black person who doesn't identify himslef with other black people as not mentally healthy, but as long as that particular person has a health self-esteem, etc., they they're mentally healthy. ------------------------------
“He may be friendly, but he's not your friend." - Malcolm X