Because there is absolutely no performance to be gained, only lost.  Both ports share the same CPU.  Sometimes sharing the CPU with the same devices on two ports will cause a slight performance degradation.
Customer is having VMAX 10K(code 5876) with 2 engines and 32 FA ports on whole. Is it a good idea to use both the ports (0 & 1) of  FA directors in same port group? For instance, I want to create a port group for a ESXi cluster with 24 hosts and use eight FA ports like: 1E [0 & 1], 2E [0 & 1] 3E [0 & 1], 4E [0 & 1].  The old directors are running on Fabric A and Even directors are running on Fabric B. The port group will still have good redundancy as it is running through, 2 Engines and 4 directors. I think we can analysis the performance easily on Unisphere under Real Time and Diagnostic. Any thoughts?
Is it a good idea to use both the ports of a FA in same port group?