I just don't see any overreaching benefits to CS's. You pump money into them, for what? To trade for a resource you could find or trade for elsewhere, at no GP cost? You can choose policies that reduce the deterioration in relations, but IMHO the precious CP's are better spent elsewhere. You could be a civ like the Thai, who have CS benefits, but I'd rather be someone else. I'd rather hoard my gold and spend it on unit upgrades. You can ally with them, but what purpose does that serve?