Actually, under Hadrian and beyond I believe senatus consultata carried the formal weight of law. Of course, I doubt any consultata were passed without the consent of the Princeps.
See above. Under Hadrian, the Senate's decrees were formally treated as law. Though of course I'm sure the Princeps control of the military actively influenced the law. ;-)
But nonetheless, until the Crisis of the Third Century and its Aftermath, individual Senators occupied nearly all the important posts in the empire. The Senate as a body was little more than a social club for the elite, but from it's ranks were drawn the most important people serving under the Princeps. It's importance really only declined when the reforms of Diocletian and Constantine invested power in the imperial court and its horde of Counts.