[quote name='Lone_Prodigy']I have a GBA and a Gamecube, and I'm just wondering if I should get the GC/GBA cable. I see the list of games compatible with it, but do I need the GBA games as well? Without them, is it worth having? Thanks.[/QUOTE]
There are two ways to look at it. Unless you've got a specific use for it in mind, it's not a vital accessory...but it's also going to be dirt cheap by now and there are some games that make nice usage of it.
[quote name='Iron Clad Burrito']It has uses in Zelda: WindWaker, but it's not mandatory. Also Metroid Prime, but I think you need Metroid Fusion to access the extra stuff.[/QUOTE]
To go into a bit more detail, linking it to Wind Waker will give you the Tingle Tuner, which is a map / quick-access store / hint device, and is pretty darn useful. With Metroid Prime, you can link the games to unlock more art in Fusion's galleries and the original Metroid as a bonus feature in Prime.
I'm pretty sure it's got uses in Pokémon XD as well, if you want to trade GBA Pokémon to the Cube game. It's also a necessity for Final Fantasy Crystal Chronicles.