Who would spend their dollars on series B bitcoin, what scenario do you envisage where someone would be tricked into buying from a second chain.
The "series B" chain could fork from the main chain a block that was posted on Jan 2013, before (hopefully) the MtGOX heist. Transacting on that chain would be like traveling back in time and doing whatever one wants with one's coins from that point on. I am not saying that such an "advantage" would be enough to steal a significant portion of the "series A" market cap, but it is one thing that could attract users to the "series B" chain.