I bought some protectors for GBA, PS1, and my NTSC-J Animal Crossing. All of the cardboard on the GBA games are in pretty rough condition not going to lie, but like sworddude said, might as well preserve what's left. Before protecting them they wouldn't hold their shape on the shelf at all, but now I can stack them up without it looking terrible.
The PS1 protectors are a bit overkill but the jewel cases are very prone to getting scratched, scuffed, and cracked so might as well protect them too. I'm sure if you merely glanced at a PS1 game the wrong way, the cover would get scuffed somehow. You can probably just buy replacement jewel cases for about the same price as the protectors though, although I haven't bothered to research buying those.
I got the protectors off eBay and Amazon, because some other sites were out of stock at the time and had no shipping estimate. So far it's been a 100% positive experience. Would buy more if they weren't so expensive!