Nothing.
It's how my car has been setup for the last year. I have cats on my car, so i rebuilt my smog pump, blasted it, painted it and remounted it with all new hoses.
I just haven't bought the belt yet to get it functional. The entire setup weighs 6.5 pounds...and prob takes 0.25HP to spin.
The best way to remove the pump is a short belt. No need for an idler pulley UNLESS you are running a supercharger kit that requires stock belt running. No point in introducing a potential pulley failure by running an idler pulley.
The cost is the same...belt vs idler pulley, but less inherent risk with a short belt vs an aftermarket idler pulley.