Agreed. I always wished Dropbox had a lower, entry-level tier for around $5/month. I don't need 50gb. I just need 5-10gb.
Maybe it would also erode the magic in it - people would start thinking if they should have this and that files on Dropbox or not. Or accidentally piling up huge bills doing some mistake managing their files...
I trust Dropbox has done extensive market research and concluded that having two price tiers, plus the other benefits that come with such simplified pricing, outweigh the flexibility of usage-based pricing.