I did find something interesting turning on my GBA with the adapter attached and no game inserted, a program loads up that seems to be the precursor to DS download play, but I don't think anything was really done with it.
If you try to play any multiplayer GBA game that doesn't require two game cards, that's what it does. I mostly see it while loading the Tingle Tuner for The Wind Waker.
I still do a lot of Gen3 trading with the link cable. I have huge libraries of pokes on those games that I call upon whenever I need a parent for breeding in current gen games (I need to trade to get more than 6 pokes across in a day, but I'm still bottlenecked by Gen4. I only have 2 pearls, 1 diamond, 1 platinum and 1 HG to work with).
I never trade over WFC though. It feels almost like cheating when trying to find stuff for my pokedex, and there's always the risk that some scumbag will give me a hacked pokemon, in which case it really will be cheating.
So yeah, I still have a link cable, set of wireless adapters, 2 GBA SPs, and a GBA player. Good stuff.