The only real disadvantage to using a SCSI2SD in this scenario is that the Beige G3 can take some advantage of newer disks with better performance on its IDE bus and/or coming off of its IDE slots.(1)
If the machines boot up in the morning and run one program all day, that benefit isn't going to be noticed, I suspect.
You might be able to use the energy saver control panel to schedule a boot up a few minutes before you expect anyone to need the machine and then put the application you use in the startup items folder so the app you want is running when you open the door and turn on the lights.
(1) As a footnote: I say "some" because in my experience the Classic Mac OS (7/8/9) is a flaming pile and its performance doesn't really scale upward with better storage. Largely, the newer and faster an OS9 machine you run, the more likely it is to do stuff like just stall out at random points or rapidly switch between feeling really fast and feeling impossibly slow. This is why most of the time when people talk about decking out Power Mac G4s, my comment is that what they're really building is something that would be great at running all the same software you could get in the late OS9 era, more reliably and faster overall, on OS X 10.3/4/5.
I get that Mac OS 9 is a unique experience, even if you run literally all the same software on it - I even prefer it, I just think it's not worth wasting the money on upgrades that don't really improve the experience. The same applies to having a lot of RAM and the same mostly applies to really fast processors. And Ethernet upgrades, to be perfectly honest.