Couple of thoughts on the one hive vs. two hive thing...
Even though two hives would be easier to keep, the number of bees that would be roaming the neighborhood would double between one and two hives. That increases the risk that one of the neighbor's kids will get stung, and that ultimately they would blame me for it.
Another thought is that the more hives you have in one location, the more competition there is for food sources, so barring catastrophe from a dead queen, etc... the one hive could possibly be stronger than individual hives in a multiple hive aviary.
Not only would the competition for food decrease, but I would think that pests such as wax moths and the like, would find it much more difficult to feed and reproduce... the same way having 1 or two cabbages in the garden won't support a large cabbage butterfly population, but a whole field of cabbages will and you'll see population explosions of cabbage butterflies to the point that you have to spray to control them or take massive crop losses.
As far as the bees pollinating my garden is concerned... having a hive, CAN'T be any worse pollination-wise, than not having any bees in the area. Even if they don't pollinate everything, they'll pollinate something. I have an apple tree and two nectarine trees too... in addition to all kinds of dfferent vegetables... so I'm sure I'll see some kind of pollination benefits... especially since I'm about to tripple the size of my veggie gardens... and one day, when we can afford it, I'll buy a farm (hopefully before I buy THE farm) and grow enough to meet all our food needs.