It's hardly convenient for the White House to see Pakistan's
prime minister* president on the brink of toppling just as Obama rolls out his new(er) strategy for Afghanistan, which we care about in large part because we want to see a stable Pakistan. That said, Zardari has been a hapless and ineffectual prime minister president, and it seems that the military basically holds Pakistan together anyway. Dictatorship under Musharaff wasn't exactly a noble condition, but democracy hasn't fared much better.
*Correction: In haste, I called Zardari Pakistan's prime minister; I know better than that. Apologies.