Life on Mars?

Acutually, there are many technological and cost-based hurdles with trying to ship rocks back from mars. First off, the rover is a realitively small cargo, so the rocket, fuel, etc is cheap compared to manned missions. The second is that they are essentially dropped in inflatable ballons for landing. There they stay.
To retrieve rocks and ship them home, you would need to carry a second rocket, or more fuel for a return trip, you would need to add a launch vehicle that could land the rover, collect the rocks, and escape mars's gravitywell. This adds additionaly technical problems. It needs to mate again with its transportation back home. Oh, having a large lander that can relaunch requires more fuel to initially launch and travel to mars, alot more. Then it has to be able to navigate home, and somehow be rocovered, either in orbit, or have some additional system to safely survive rentry.
One way trips are way cheaper, simplier, and lighter. The huge difference of price also allows you to fund many other projects for the money of just one return trip.