> The PS of my cab is dying and I thought to replace it with a spare (switched)
> non-ATX PC power supply.
>
> Is there a reason not to use a PC power supply with arcade boards?

I can't think of any. I used this as a guide when I built my JAMMA rig, check out the PSU information section.
http://chadsarcade.co.uk/oldsite/Supergun/supergunIndex.htm

-Sune