1) Height 2) Use of spare limbs for tools
Height doesn't seem to be much of an advantage for robots at this stage, and I think that robots could have 6 limbs with no problem. Therefore, biped robots are really only a novelty.
Ladders, automobiles and heavy machinery, household equipment, etc. all rule out quadrupeds.
While it would be possible to custom-build all of this stuff to be usable by a quadruped, it means you then have to have two of everything if you plan to cohabitate with a robot assistant. From the perspective of assistance robots, it's not going to be possible to accomplish everything without bipedal bots.
I think wheeled bots (like Trevor's Monty) will also be a great option--probably a tenth the cost of a walking mechanism, an order of magnitude more speed and endurance, etc. But they'll still be unsuitable for many assistance robot tasks. Stairs, for example. And unlike the Daleks, leveling the building will not suffice as a solution to the problem of climbing stairs.
Who's to say a quadruped robot can't have 4 feet + 1 or 2 grapsing mechanisms? Or have two front feet that are also capable of grasping?
You and I might think it's really cool to have 4 feet and two arms (like a centaur!), but if a robot reminds someone of a gigantic insect, and it's suppose to be a household companion/housekeeper, people might reject having it around the house. That's a good reason to have bipedal robots.
But on the other hand, if you get close, but not exactly with bipedal robots, people might get even more freaked out with the uncanny valley.
Personally, I'd love to have a robotic centaur helping me around the house.
I can see two legs being more efficient than four, but wheels are usually vastly more efficient than either..