From my vantage-point, I see every conscious human, theistic or otherwise, relying on things that can't be proven. Faith is a pervasive aspect of existence – though there are many different kinds of it. Even to say that life and humanity have value relies on things I cannot prove. 

This pervasiveness of faith to normal living has not gone unrecognized in Western philosophy. David Hume, for example, in one fell swoop, demolished classic and medieval arguments for the existence of God, alongside all scientific certainties that rely on assumptions from inductive reasoning – and simultaneously demonstrated that an act of faith (relying on the occurrence of something that can't ever be proven as certain) is involved in every single instant of normal living. Far from being a criticism, though, under normal circumstances, I consider such believing necessary and appropriate. I think Alvin Plantinga, with his articulation of the notion of "properly basic beliefs," has some reasonable things to say along these lines (though I'm not invested in the exact outcomes of this process for him).

If we make our sole criterion for the reasonability or unreasonability of faith to be whether something can be proven, then I think we all fall flat: myself, you, Sam Harris, ISIS. We need to be able to make distinctions amongst different kinds of belief, if we are to do more than roundly shut out a whole aspect of human experience – and lose our basis for making positive or negative judgments about how it manifests. So, for example, we can can ask questions like:

1) Does the belief ask me to accept something which is irrational or fallacious? On the other hand, does it ask me to accept something which, though not strictly irrational, is not subject to empirical verification?

2) Do the things believed in contradict what I know to be true from other sources, such as that which is evident to my senses?

3) If so, do I have grounds for believing that these other sources, or my senses, may be providing misleading information?

4) If it asks me to accept something not subject to empirical verification, are the difficulties of verifying it time-conditioned, so that they may be resolved as further observations accrue? Or, is this something that can never be conclusively observed from within our ordinary space time perspective? 

5) Is the belief negative, neutral, or positive with regard to information from other sources? For example, "negative" would mean it contradicts or asks me to disregard something told through these other sources; "neutral" would be a belief that is indifferent to what I know from these other sources; "positive" would be something that enhances my engagement with what comes from these other sources. (For some, for example, the belief that God has created a material universe governed by definite laws actually incites them to pursue empirical study.)

6) Similarly, we can ask whether the belief is negative, neutral, or positive with regard to other values. For example, does the believe cause me to put aside what is considered valuable about things like life and humanity? Or, do the beliefs incite me to more fully explore and live out these values?

7) Do practices and behaviors that grow out of this belief facilitate healthy and vibrant personal, social, and ecological life, or do they create dead ends and obstructions to development in these spheres – including even the extreme possibility that they actually encourage destruction? 

This is just a start. I could come up with more points than this. The point is that I do not see the crude rejection of all faith as being useful at all towards helping us understand what factors lead us to be more or less all that we can aspire to and be as human beings.