I used my cheap-o Nashbar steel rear rack as a seat for my wife once. She went for a run and I was tired so I pedaled along to keep her company. She got a bad cramp about 1.5 miles from the house so she hopped on the back and I gave her a ride back. This was on my Trek road bike with 700x23 tires. Bike handled like crap but it worked in a pinch. Would I do it on a regular basis? No way!

Edit: she only weighs 100#.