The slot-2 Supercards, while not the greatest for GBA compatibility, remain some of the most updated slot-2 devices available. The SC team has maintained their slot-2 products with updates that add features normally found only on slot-1 devices, such as auto DLDI patching, cheat support, and in-game guide. You can still use it to play most GBA games and run the Opera web browser, so I'd suggest keeping it.
Also, if you have any intention of running DSLinux, I'd keep the Supercard for it's RAM capabilities.