Also, as noted, seems great to let your colleagues see what's going on with your work.
This is already easily done just by configuring port forwarding from your host to your guest in your vagrantfile.
e.g.:
config.vm.network :forwarded_port, host: 8080, guest: 80
So, as you can see, if vagrant share works as announced it would make it much easier for me for that particular task.
Plus if you are at the point of QA/PM review of what you've done, the code has already been checked into source control. They could just as easily spin up their own VM with the Vagrantfile.
I used Vagrant for CI builds for a while, and I would have liked the ability to automatically share an internal url to the environment so that when coworkers submitted code that broke the build, they would have a single link to follow to get access to the particular environment on the particular server that serviced their job.
Some vagrant envs are also somewhat complex to set up - I could use this for teaching people how to use things like this: https://github.com/stackforge/puppet_openstack_builder
Anecdotally, it also seems to correlate with how useful Developers usually find their peers.