Occam's Razor
Seems to me the side of the razor on which this falls depends at least partly on how likely it was that life COULD have developed on early Mars. From what I understand, the earliest signs of life on Earth appeared very soon (geologically speaking) after conditions made it at all possible, and that conditions on early Mars were likely reasonably close to those on early Earth. If the beginning of life is a good possibililty, then wouldn't biological explanations of the features seen in ALH84001 be more simple than the combination of non-biological processes that would be required to produce the same features?