[mwhafner]

One application where I joint on a router table is when working with highly figured woods where tear-out can be an issue on a standard straight-knife jointer. I use a spiral bit on my table, and that helps tremendously.

A track saw also works amazingly well to edge joint stock, in the right circumstances.

I can completely understand jointing with the router, but planing? Planers aren't that expensive are they?

No, but a planer won't flatten a board like this method will (cups, warps, etc). Think of it more like a method of face-jointing large stock.