This has always been the great debate for whether or not atom even needs to exist. The power savings difference has always been there, but the performance hasn't. Atom would no doubt be a killer cpu if it performed better, but it's kind of hard to want one if you're going to be giving up performance. If you had a P4 system it costs more to run it, but since atom isn't outperforming it by much you're not really gaining anything by upgrading it. The best way to describe it is that atom has great performance per watt, but poor performance per dollar. The SB celeron has both which is what makes it more attractive than atom.
Like NexusN said, a lot of single users will probably choose to pay a bit more for sandy bridge even if atom could do what they needed. We're all enthusists here because otherwise we would just be using a soho router that costs less and uses less power than even atom (least power and least performance). So given the choice between SNB (performance) or atom (price) it's not hard to see what people pick. If you were setting these up for a business price comes before performance so atom would likely win. If both processors were able to accomplish the job, atom is still going to do it for less. Let's say it costs $15 per year less to run atom. If I were building 100 of these setups that would mean each year it's operational we're saving $1,500. So you could lower your TCO by $4,500 over a 3 year period.