maslahah

Dear scholars, As-Salamu `alaykum! It seems that today the scholars are practicing maslaha on issues that they think benefit the Ummah. From the seerah (biography) of the Prophet (peace and blessings be upon him) it is obvious to see that such practice never took place where there was a clear hukm regarding the situation, or if a ruling can be extracted with qiyas. Now as Muslims, we don't live in this life for our benefit, be it on individual basis or the Ummah as a whole. We live strictly to gain the pleasure of Allah only. Now the question is what do these scholars base their decision or fatwa on? Is this valid? If it is so what is the daleel (evidence) for it from the Qur'an and Sunnah? Jazakum Allah khayran.