Having your uplink faster than all your individual ports in a network helps prevent any one port from being able to saturate your network.
I work at an ISP that within the last year started selling 10Gbit symmetric, and we all sorta know there's not a huge use for it - yet. It's not really an issue for use to justify though, we don't charge any extra for it, it's all $40/mo and if you're in a new area we're building you get 10Gbit instead of the old 1Gbit at that price.