I don't think anyone has claimed expansionist is useless, just weaker than all other civ abilities. The slight advantage in exploration is good only for about the first 50 turns of the game unless playing on a huge map or with few civs. The tech advantage is quickly surpassed by scientific civs, and the other civ abilities yeild significant advantages throughout the game. since you can't take advantage of you exploration because you get settlers and population at the same rate as others, and because you almost always end up trading maps in the first 2000 years anyway, expansionist just isn't very powerful, except in very specific situations. It is quite likely to get no advantage at all from the scout, if you end up being placed in a crowded environ.
I've played about 10 games with expansionists, and have got the extra settler from a hut once. The barbarians are hardly ever a threat when you find them, and I see them more as a chance to promote units.
Expansionist isn't just building lots of cities, its quickly expanding those cities. Granaries make perfect sense for expansionist mindsets, as they let you exploit your surroundings faster and build new settlers and workers faster.