From my perspective, I think it benefits the Islamic extremists to have pulled this off. First of all, they know that they can interfere with Musharaff's power by perpetrating this act and turn even more of the populice against him. Keep in mind, Musharaff has been a thorn in the side of the extremists for a long time - waging an internal war against them on behalf of the U.S.A. Secondly, Bhutto, as a leader, would also be bad for the extremists as she supported popular "reform" which is "UnIslamic" according to the extremists. Given the recent strife between Bhutto and Musharaff, the extremists knew that they would be able to plant seeds of doubt among the population regarding responsibility for her asassination.
Two birds, one stone. This act will continue the erosion of Musharaff's power and has eliminated another potential political rival to the
extremists. The ensuing chaos is likely to unsettle the entire nation for the forseeable future.