One of the advantages of an odd waistcoat is the visual change of pace it provides. The gray suit with a gray vest is one look. The same jacket and trousers paired with a complementary waistcoat is another and is an addition that works well with all those ready to wear single breasted jackets that come without waistcoats in the first place.
The best odd waistcoats to my mind are linen for warm days, tattersall for not terribly formal occasions and cashmere when you want to look your best (waistcoats use only half a yard of the stuff, making it a relatively economical way to add cashmere to a wardrobe). Buff and cream versions in those fabrics are the most useful, complementing as they do both gray and navy blue jackets.