In several states this is considered illegal. Although, it's unlikely you will be caught.
Regardless, if the network is unsecured, then all of your data is being transmitted in plain text which allows others to read every single solitary website you visit, comments you post, IM conversations, emails you send and receive, and user names and passwords - get the point? You don't even have to be associated with the access point to read everything that is sent in plain text - you simply need to enable monitor mode on your wifi card using specialized software, and you too can see everything that noobs are transmitting in the air. WEP is equally worthless.
Additionally, this significantly opens your computer up to attack and compromise. Personally, I never authenticate to a network unless it's encrypted using WPA2.