Horizontal proliferation among Muslim countries will ensure a larger sized collective nuclear arsenal that can be used against common "enemies of the Islamic world", countries like Israel or India, and can also provide the strategic depth (to protect their weapons programs and stockpile) and the geographical reach to deploy nuclear warheads (with limited range) close to their target.
Very soon it will become a chain reaction as one by one Muslim countries will unite to become nuclear powers. Not only will the Islamic world assume greater political clout and resort of a global nuclear blackmail, the risk of nuclear terrorism will increase greatly.