Starting is always the hardest time on your engine, and is when the most bearing wear occurs. When your engine is running, the crank and cam don't touch the bearing surfaces. The oil is forced inbetween the bearings and the crank, creating a "wedge" of oil that keeps them from contacting each other. This wedge needs sufficient rpms to work. So even though your pressure may develop when starting, you still have the crank rubbing against the bearings. More so true with the crank than the cam. Cam bearings can last forever. This really only applies to the main and rod bearings. Anyway, when you are starting the engine, the rpms aren't enough to protect the bearings. Once it fires, and the pressure is still up, the bearings are safe, until the next time you try to start it.
Take care,
~Chris