as said, there could be a few reasons not to run it/leave it unplugged.
If the distributor has been set up with an aggresive curve, or for racing only, it would normally be unplugged. might ping under light load otherwise.
I use a delete plate to cover the hole, and still have a tab to line the cap up with, but if you didn't have that, you would likely have to put the vac pod back on, so the cap would have an index.
On most street cars, there is no real benefit to not running vac advance, BUT equally if your distributor has been set up properly it's all out performance will not be any different with the unit unplugged.
It shouldn't even affect your idle, as ported is the correct source for vac advance. It should only add timing under light cruise.
