When your revenues scale with the services rendered, it *does* make business sense to auto-scale. Auto-scaling is a technical solution, not a business one.
Being Slashdotted isn't typically associated with more commercial activity, it's associated with "hit-and-run" visitors. The same with social networks. Does Twitter even have a business model?
But wherever there's a business model where margins are relatively stable but activity rises and falls, auto-scaling makes you money rather than costing you severely.
Like many things, it's a tool which should be used wisely, where not paying attention can leave you missing fingers.