It's more difficult to split your work across 6 GPUs evenly, and easier when you have 4 or 8 GPUs. The latter setups have powers of 2, which for example, can evenly divide a 2D or 3D grid, but 6 GPUs are awkward to program. Thus, the OP argues that a 6-GPU setup is highly suboptimal for many existing applications and there's no point to pay more for the extra 2.