While it is a fair assessment to note that "history is written by the victors", and thus we can recognize the biases of history; but it is quite another thing to simply throw all the history books in the fire and simply make up an alternative story that fits whatever one wants.
Are historical sources biased? Of course they are. Every work written by human beings contains a bias. But the alternative to biased history isn't burning the history books, but taking such bias into account in our study of history.
If one were to take two contemporary works on the war which led to the thirteen colonies in America becoming independent, one British and one American, we will have two biased works of history. That, however, doesn't make either work useless; but rather provides us with the perspectives of those writing the work, but still writing about real things that happened.
Bias does not eliminate the value of history; it is simply something to be consciously aware of when doing historical study.
So, as an example, are official Church sources going to have a bias when writing about certain groups (e.g. ancient Gnostics, medieval Cathars, et al) of course. But that doesn't mean that we can just make up whatever we want about those groups to suit whatever agenda we might want.
Of course official sources will depict the Cathars in a negative light, but that doesn't mean that the Cathars were actually Baptists, their rites, their beliefs, their practices, etc can still be ascertained. And ascertaining that, it's clear that no Baptist today would want to be associated with the Cathars, or with the Bogomils, or with the Paulicians, etc.
Bias is not an excuse to throw history books in fire. Again, bias is simply something the honest historian has to take into account when studying history.
-CryptoLutheran