It depends on what games you want to play.
Friend codes help you to connect to folks, but they are not the deal breaker they are portrayed to be -- most (if not all) DS online games let you play with completely random strangers, just like in the PC world. I think most complaints come from folks spoiled by the wonderful, feature rich, XBox Live service. Some games allow voice chat if you have traded friend codes.
Metroid Prime Hunters online is a blast, and there are plenty of people playing. I've enjoyed Space Invaders Extreme, and Starfox Command still has a few people playing. Some games like Pokemon Ranger Shadows of Almia and Professor Layton have new levels you can download. Pokemon Diamond/Pearl/Platinum allow you to trade Pokemon online, and you can battle other players. Harvest Moon Island of Happiness has leader boards for weekly tasks/competitions. Animal Crossing Wide World has some events that only occur if u have online connectivity. You can download your friends creations in Spore Creatures.
I do think it enhances your experience, especially for games you are passionate about.
As far as my WEP complaints, from an experience perspective, you will notice no difference from WPA -- my concern with WEP is that there are lots of hacker tools that people can use to break WEP security and get access to your network -- but they would have to be living next to you, and hanging out in front of your house to be close enough to grab your signal.