The benefit of a splayed cap I believe ,is that it pulls together the pan rails,helping to stop a crack forming from the mains saddle oil holes towards the cam tunnel. If you look at the way the forces from a descending piston are applied to the block, you will see that the force applied pushes at an angle towards the pan rails,these constant,oscillating forces flex one side of the main saddle towards the pan rail ,and are then reversed when the opposite side piston descends and flexes the other side of the main saddle towards the opposite pan rail. All that is stopping this side to side flex are the main caps locked against the dowels. A splayed cap pulls the pan rails towards the mains saddles and against the dowels, effectively 'squeezing' the main saddle together. Hence why Dart etc aftermarket blocks still use just the economical splayed caps when they could go to any configuration they want,ie,crossbolted,which may be better but is almost certainly more expensive to manufacture. Of course,the actual thickness of the area under the main saddle and out towards the pan rail has a direct effect on things, which is why the early blocks have a reputation for being stronger,they simply have more 'meat' in these areas(not to mention the side rib).
This direction of the applied forces is why a straight 4 cylinder block can take such a massive increase in power(compared to a production V8) without cracking through the mains.
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