So, by creating a new branch allowing the people to vote directly everything all the time, you'd obtain two things:
a) Media would decide whether laws pass or not. A regular person does not have time to read every single line in a proposed bill, a representative does because that's his job, after all. Regular people do not care about politics in general, representatives do (because bla bla bla). So, in other words, they'd just watch TV and hear the Fox or the CNN folks saying "Yes!" or "No!" and vote accordingly.
But hey! That's what we have now, don't we? Well, there is a difference. Professional politicians try (or at least should >_>) to find agreements between all the sides to pass on legislation, and can rewrite and discuss it a million times until they are happy with the result. But if people are just given a closed, already written law to say yes or no, they lose the major part of politics: discussion. Unless they vote no for everything and get everything to be rewritten completely until "the people" are happy with it.
b) Laws would take waaay longer to be passed. If they already take a hell of a long time, now add a popular referendum and, of course, the chance of getting a "no", sending everything back to the starting point. A country can't afford that nowadays.
Not to mention that we just can't expect everybody to be interested in politics, and forcing people to take a test to make sure they are up-to-date about politics to let them vote would only create a new elite (so... there goes the plan to let everybody take part in politics), because, as I said, some people... just don't care. And a mobocracy isn't the best choice out there, really.