Other things that come to mind:
Pair production requires that you create just as much antimatter as matter. You need to have some way to store all that antimatter you just made, which incidentally explodes on contact with everything. On the upside, you have a quick way of recovering some of those quintillions of joules of energy you used to create you matter.
Once you made your elemental particles, you need some way to assemble them into atoms and then the atoms into molecules and then the molecules into whatever it is you're trying to build. If you have a way to do that (nanotech, presumably), it strikes me as much simpler to just take the atoms and molecules that already exist and use that in your holodeck rather than bothering to spend all the energy in the world.
Don't get me wrong, the whole thing is great news from a theoretical physics research point of view. I just don't think "holodeck" is a realistic practical application.
