Common sense (and engineering) will tell you that if you have a pipe with leaks, putting a cap on one leak simply drives the oil out the other leaks. From an environmental perspective this makes no sense, because the oil is still leaking out of the well, just in a different place. Of greater concern is that the oil can flow out into the surrounding rocks the way the mud did during the failed Top Kill procedure, forming what is called a subsurface blowout. This is a huge bubble of oil inside the rock. The danger is if the bubble ruptures and the oil collected over 80 days of leakage rushed to the surface of the gulf all at once. Such a surge of oil could easily engulf and founder the surface ships.
Capping a well with known leaks below the surface really serves only one purpose, and that is to present to the media video images showing the well is capped, so that they can reassure the public the problem is over and please vote Democratic-or-you-are-just-being-a-racist.
But the hard reality being withheld from the public is that the problem is not fixed. The leak has not been stopped, it has just been moved out of sight. The camera feed of the capped well head will be shown constantly to assure Americans that all is well and lull us back to a useful torpor. No other views from around the well will be allowed.
Except that the cat is already out of the bag!