Is there any point in enabling QoS on a dedicated VOIP network that only has VOIP traffic?
No, there is absolutely no reason why you would if you are on an isolated network. Quality of Service is simply a queueing mechanism that prioritize traffic based on predetermined requirements. If you enabled QoS on this network, you would effectively be saying "guarantee 100% of my already 100% available bandwidth is guaranteed for VoIP".
Do you need QOS on a dedicated VoIP network?
Possibly not, but you probably want it in place.
It really will depend on the network layout/capacity, the complexity of your phone system, the volume of voice traffic, and your business needs.
Here is the most basic example I can think of:
If a link is so saturated with calls that call-control traffic can't get through, what happens? You should ideally still use QOS to give call-control traffic a guaranteed portion of each link. If you do not, you could potentially wind up with all sorts of unexpected call behavior. (Dropped calls, one way traffic, etc, etc)
Another consideration is, do you control the entire network? If this is multiple sites connected across some sort of MPLS service, you may need QOS to prioritize your traffic within the provider's network. (Which they'll gladly sell you at a cost.)
The bottom line is this:
Do you care about a specific portion of your network traffic reaching it's destination? Then QOS is your friend.