Well, WEP is marginally better than nothing so if you're gonna use wireless without WPA, use WEB rather than leaving it open. That way at least you don't have the casual neighbours connecting and stealing your bandwidth. Keeping your SSID private and limiting access to certain MAC addresses will help to eliminate opportunistic "hacking" with iPhone apps and the likes.
I haven't heard about the WPA-capable driver, but isn't WPA dependent on the CPU for some of the encryption? Might not be fun on an already slow machine.
As for the "Why?" question, I use Wifi on my A1200s for transferring files via FTP, works a treat! With most of the WWW depending on CSS, JavaScript etc., browsing is next to useless unless you use it just for downloading from Aminet. 10 years ago it was bearable, not any more.