If you're using a JAMMA wired cabinet with an arcade CRT monitor and you send the signal through a J-PAC, you don't need to use Atom-15. The J-PAC will divide out-of-range signals to protect the monitor.
But for people who aren't using a J-PAC, Atom-15 is probably a good idea.
Note that when using Atom-15 you cannot view the BIOS on most modern PC motherboards that use a high-res UEFI BIOS.
Though, it's not a huge deal. You can always remove your Radeon card and use the on-board video connected to an LCD for accessing the BIOS.