Some random guy on the Internet says:

Contrary to what most of us would think as logical to stop the oil mess, actually opening up the gushing well and making it gush more became direction BP took after confirming that there was a leak. In fact if you note their actions, that should become clear. They have shifted from stopping or restricting the gusher to opening it up and catching it. This only makes sense if they want to relieve pressure at the leak hidden down below the seabed..... and that sort of leak is one of the most dangerous and potentially damaging kind of leak there could be. It is also inaccessible which compounds our problems. There is no way to stop that leak from above, all they can do is relieve the pressure on it and the only way to do that right now is to open up the nozzle above and gush more oil into the gulf and hopefully catch it, which they have done, they just neglected to tell us why, gee thanks. [...]

This down hole leak will undermine the foundation of the seabed in and around the well area. It also weakens the only thing holding up the massive Blow Out Preventer's immense bulk of 450 tons. [...] If you have been watching the live feed cams you may have noticed that some of the ROVs are using an inclinometer... an inclinometer is an instrument that measures "Incline" or tilt. The BOP is not supposed to be tilting... and after the riser clip off operation it has begun to... [...]

What eventually will happen is that the blow out preventer will literally tip over [... ] as the well casings cannot support the weight of the massive system above with out the cement bond to the earth and that bond is being eroded away. When enough is eroded away the casings will buckle and the BOP will collapse the well. [...]

All of these things lead to only one place, a fully wide open well bore directly to the oil deposit... after that, it goes into the realm of "the worst things you can think of". The well may come completely apart as the inner liners fail. [...] the very least damaging outcome as bad as it is, is that we are stuck with a wide open gusher blowing out 150,000 barrels a day of raw oil or more. There isn't any "cap dome" or any other such fixer device on earth that exists or could be built that will stop it from gushing out and doing more and more damage to the gulf. While at the same time also doing more damage to the well, making the chance of halting it with a kill from the bottom up less and less likely to work, which as it stands now?....is the only real chance we have left to stop it all.