Besides, voting based on what most vocal/active constituents want is usually a pretty good idea, because they are the ones who usually research the topic the most, and have good solutions, and want their voices heard. And a senator would still have to make a judgement call and think about the whole population he's representing.
Until you get voting rings going, in which case it's up to raising awareness about an issue and that's out of the domain of such a system, is it not?
Faulty syllogism there, in my view. The most vocal/active constituents can also be the most easily manipulated or the most extremist ones. Consider that there have been several Congressional attempts at immigration reform in the last decade or so, all of which have been derailed by a hardline nativist lobby which insists on deportation of all illegal aliens, which nobody in their right mind considers sensible, ethical, or practical.
Frankly, I think there's an inverse correlation between political passion and expert knowledge.