SQLNET.EXPIRE_TIME will kill inactive connections. However, it will not kill them until the connection requests to do something.

For example, say you have your SQLNET.EXPIRE_TIME set to 10 minutes. A connection will still show up in v$session until 10 minutes has passed AND somebody tries to execute something on that connection. If the session is really idle, it could sit for days without anybody executing anything. Then when they do, they'll be pissed off because their connection died.

Setting up shared servers would be a better use of your memory, IMHO.