Cross drilling is overkill for a non-tracked engine, especially if you're spinning it below 7000 rpm. There are tons of people who track/race their engines and the lack of cross-drilling isn't blowing these engines up regularly. It's only for the high revving engines for added insurance in getting the oil to the #2 and #5 rod bearings more effectively.
The theory/reasoning behind the cross-drill is because it is a known fact the crankshaft is fed oil from the ends. Main bearings #8 and #1 feed oil into the crankshaft and that's the source of oil for the rod journals. Since #2 and #5 rod journals are the centrally located rods, they get oil last. If oil demand/loss is high on the outer journals, then the inner journals like #2, #5 are going to suffer.
Also noteworthy is the #1 main journal is grooved, which helps the oil find its way into the drilling in the journal. #8 is not drilled I assume because the bearing insert is much wider and has a better ability to feed oil into the crank at that end? Not sure there.
The #4 bearing hole, and of equal importance is the hole in the case saddle leading all the way to the oil gallery inside the case, is enlarged to ensure the cross drill effect is taken advantage of. The larger hole is the path of lesser resistance and that means the oil path is going to be activated more easily than the surrounding paths that have smaller holes leading from the case main gallery to the main bearing saddles.
Here's a good thread with some pictures to describe what's going on with the oil path
http://forums.pelicanparts.com/911-engine-rebuilding-forum/582197-another-crank-cross-drill-question.html