Oblivion did not have this behaviour. I see no reason why it should be neccessary now. Besides, i should be able to set the intervals and see how my system handles it. Even if just to show me that it would slow down my system too much, which i still don't believe it would. There's some other reason for this.
I really don't believe there is. As far as how to set the intervals, I'm betting that may be abstracted by the shadow quality setting. Crank it all the way up and see if it changes anything. I'm not saying it will. Like you, I don't really know why they did it that way, but having experience in game development, I can make a pretty educated guess (as in, I am 100% positive) that they don't employ real-time shadows to save on processing overhead. There would be no reason not to use them otherwise.
As far as how Oblivion did it, I don't remember ever noticing how the shadows behaved. It is conceivable they had more processing budget available for real-time shadows, or they may have used another technique, like blending.