Why the Dispenser?
When you switch your weapons, the resupply cabinet in tf2 spawns weapon pickups on you (similar to hl2 or portal to whatever) that are only active for one tick and can only go to you. If something blocks those weapon pickups from getting to you, you can end up with no weapons.
And John, it is also possible on mvm_rottenburg (the forward upgrade station) and pl_frontier (final red spawn).
I've learned a lot about tf2 from finding these exploits.