does my investment affect my belief? maybe if i weren't invested in it i wouldnt believe?
OK, I'm a philosophical neophyte, but my 2p's worth:
Speaking personally, I think beliefs are fundamentally aesthetic choices; we find statements about the world (and those axioms we use in filtering such statements, for that matter) elegant or inelegant, pleasing or not pleasing (which to my mind also introduces indifference as a classification). To that extent, the truth of a statement is relative to the individual contemplating a statement; the rigour by which a statement is assessed becomes a matter of personal choice, and the consistency of our beliefs suffers or benefits accordingly.
To return to your question; your investment in knowledge is one basis from which you argue for knowledge, but I don't think disinterestedness in a belief is essential to argue for or against it. The principle of charity only suggests that you give the best possible interpretation of a belief before you can attack it, no?