Seriously, there is absolutely no reason to buy vouchers apart from using one to make the S+ run on Professional a bit less difficult. The key point being that even with an unlimited ammo weapon from the start that run is still extremely challenging and yes, frustrating.
It takes a bit off the grind, but realistically most people will need to do a bunch of full-story runs in order to prepare anyway, and along the way you'll most likely easily unlock all the other infinite use weapons.
The bottom line is that playing for that run is not for everyone. Its the definition of a task for hardcore completionists only because it not only takes skill, but it requires a ton of planning due to the limitations to both time and number of saves.
From a practical standpoint most players can just do multiple new-game+ cycles and unlock the crucial stuff that way, and by crucial I mean stuff like Ashley's special outfit that is almost mandatory for speedrunning!