Just my point of view :
Necessary? No.
Useful? Yes.
I bought my X-Pro in 2012 because I wanted both the optical and the electronic viewfinder. Today I just love the EVF and I have to admit I use it 99% of the time.
But once in a while I'm very glad to still have an OVF, specially when it comes to improve the battery life of my X-Pro: when an full EVF body will just die (X-E, X-T, etc), the X-Pro or X100 will save you a few more shots. Very helpful sometimes.
Also in some situations (like street photography), the OVF allows you to anticipate your shot as you can see what is happening outside of the frame in the viewfinder. So yes there is a parallax error, but if you know your lens it won't a big issue anymore.