No they shouldn't, which is why they're where they are in the polls.
It takes a lot more than ideological purity to appeal to the vast spread of center-right voters. Things like demonstrated executive capability, and a personality that compels others to follow, are prime requisites to capture sufficient support to win the nomination.
There are many other intangibles, and the laundry list of skill sets and attributes is long, but the point is, there's a 'total package' that center-right voters are looking for in their next president. Neither Bachmann or Santorum score well on that list, which is why neither of them will be the nominee.
There's even a chance that Romney and Gingrich will fail to make the cut with the voters. It's a wild ride we're on, and anything can happen.
I think of that when I look at all the naked car bumpers in my neighborhood.
These people don't want BO.
They don't know who they DO want.
Yet.