Personaly i don't think there is much need for civilian units for every specialist and most are better represented as they are, inside the cities. I see the case for a unit mostly in interaction with other civs, with the spy and potentialy with a diplomatic unit(which i definitely support). The merchant obviously interacts also with other civs but is better represented through traderoutes in that regard than through a unit. The missionary gives you control over religion that you in another way just dont have. A rethink of the specialist boni on the other hand seems kinda interestings. For exampel the maintenance bonus could be interesting on statesman specialist itself. Or an boni to traderoutes from the merchant.