It matters because without it, what you say (and do) means nothing.
If you say one thing, and do another, you're a hypocrite.
If you advocate for a policy, and vote against it, you can't be trusted.
And all too often, consistency can't be felt, seen, or heard.
Specific to politics and faith, I find an increasingly inconsistent pattern among right-wing-voting-Christians (which is how the majority of Christians vote - Republican). This inconsistency is branded across the Church and perception becomes reality - that Christians are either ill-informed, can't make a consistent and intelligent argument, or are just plain hypocritical.
On one hand, Christians will overwhelmingly support other Christians in politics (usually right-wing), so that politics resemble the ways of Jesus, biblical truths, etc. They believe that electing more christians will make our nation Christian once again (essentially that Christian leaders will enact christian laws).
And most Christians will also agree that Jesus came to earth, and lived his life to show grace and mercy to those that didn't deserve it (i.e. sinners, the sick, etc.), and that he came first for the marginalized, the poor, the blind, and "the least of these."
However, when the government enacts policies and social programs that extend similar grace and mercy to the least of these, Christians overwhelmingly call them "entitlements" and say that the government has no business in these matters. So either the government is or isn't in the position to act and dispense policy, based on the Christian faith. They cannot have it both ways, because consistency matters.
In the same light, liberals (which most overwhelmingly support our President, and Democrats) cannot remain consistent in their anti-gun/violence messages, while also remaining silent when the government is using drones attacks at an alarming rate and when our policies place assault rifles into the hands of Mexican drug lords. The liberal message of non-violence has no backbone.
My point is this...there must be a true separation between church and state. This is a good thing. Because when the perceived "Christian" candidate on one hand argues for life in repealing abortion rights, yet funds the military and supports capital punishment on the other, there is no consistency. Likewise, when a liberal fights to pass laws that control the sales of arms (to "save even one life"), but systematically places these same weapons into the hands of Mexican drug lords, there is no consistency.
In both scenarios, I believe that arguing for life and saving life most closely represents the way of Jesus. And in both scenarios the politics of the state become the voice of the church - either in support or criticism. By separating faith and politics, the United States is not the mouthpiece of Christianity; and when it behaves in less than Christ-like manners, one is not co-opted by the other, and consistency is not in question.
Because no matter which side you find yourself, consistency is lacking - and when left unchecked, both sides become impotent to truth-telling and agenda. Consistency matters because without we nullify any resemblance of intellectual honesty and debate. And without consistency, we all become hypocrites.