Brad, consider these two scenarios:
1) Someone wants free access so they crack your WEP encryption and spoof a MAC address to nick your wifi connection. You may or may not notice, but the end result probably isn't too significant (unless they're a super leech)
2) Someone wants to see your data, so they capture everything and crack your WEP encryption. Once WEP is cracked they can go back to the previously captured packets and view everything unencrypted.
Because it's 100% passive the first you may know of this is when they actually do something with the data they've captured. This scenario could be far more damaging to your business and wellbeing in the long run.
WEP is junk, you need something far stronger.