Uncertainty about the economy generally and uncertainty about tax rates are two separate and distinct things. First, there is no real uncertainty about taxes. Everyone knows full well what happens in two years. Taxes go up. To the extent there is any uncertainty, these businesses that are so concerned about uncertainty can avoid that uncertainty by making clear to their members of Congress that they would prefer the increased rates of current law to uncertainty attendant to changing the law.
People aren't hiring because the economy sucks and shows little sign of significant improvement in the near term, not because marginal tax rates might increase a few percentage points in a few years.
The economy sucks and shows little signs of significant improvement because we are $13 trillion in debt, and Congress continues to spend money at an alarming rate! I've not heard the first Democrat talk about seriously cutting spending, or cutting the budget. All Democrats seem to want to do, is add new government programs, and spend even more money we don't have.
With taxes and economic stimulation, it's really simple... the more money people get to keep in their pockets, the more money they have available to stimulate the economy. Raising taxes helps to stimulate the economy, like water helps to build a fire!
Now, there are a bazillion reasons people hire. There is no way to say that lowering tax rates will certainly cause people to hire, but it can indeed be a factor. It's not just from a monetary standpoint, because they will save $5k on taxes next year, they can afford to hire a new employee... that's static economic thinking again, which pinheads are famous for. When money is returned to the people, they tend to spend it. When they do, business increases, orders increase, profits begin to rise. As this all starts to take off, the astute businessperson might seize the opportunity to expand, anticipating great returns on their investment, because business is coming back. It is the optimism created by spurred economic activity, caused by tax cuts, which may or may not create new jobs, depending on the business. Some businesses do not prosper well in a vibrant economy... title loans... pawn shops... a tax cut and economic prosperity, might actually cause them to have to lay people off. The point is, nothing is static... everything is constantly changing. Regardless of any argument you can make, tax cuts always spark economic growth and activity.