Let's just ignore whether you need it or not for a moment. I'd say one of the defining aspects of this console generation has been confusion. Nintendo started things off with the Wii U. A console that presented confusion in the form of whether it was a new console or simply a controller upgrade for the Wii. It's something that they continually had to deal with. Then Microsoft had major issues with their DRM reversal. People mention even now in stores there are customers who still think that the Xbox One has DRM. Sony is the one company that managed to avoid the confusion. This is due to how simple their approach was. While Nintendo and MS were trying to push different things, Sony was basically like "Here's the follow up to the PS3". Nothing complicated.
So now we potentially enter this thing in year three of the PS4's life. If the rumors from Zoetis are current then it's significantly more powerful than the current PS4. Sony's going to need to communicate that power gap. But in doing so it's bound to lead to lead to confusion. On top of that they're going to have to communicate about PSVR. Yoshida mentioned this at GDC
http://www.theverge.com/2016/3/16/11242372/playstation-vr-shuhei-yoshida-interview
That's how easy it is to confuse the general public when it comes to electronics. So they already have something lined up this fall that they already know is going to be a pain to explain to the general public. Adding a new version of the PS4 to the mix is only going to complicate things more as far as clearly explaining the differences between them and how they all work.
Aside from the confusion aspect, they open up another problem by having people that are inevitably going to want to choose between PS4.5 and PSVR. On one hand you have this new way of playing games, but on the other you get a significantly more powerful console. Given how hard it is going to be for VR in general to catch on, why would Sony want to damage its chances by launching a new version of the PS4 around it?