As a couple of posts above say:

a lack of lights could mean the link is out, OR the indicators have packed up.

and


So even when a robot in the arena shows no lights stay carefull.

If you have put your link in your robot you should treat it as live at all times, regardless of what indicator lights say. Given that, it seems to me that a power light is as likely to cause accidents as to prevent them because it could give a false sense of security in some cases. The best indicator of a robots status is the link itself - if it is in your pocket the machine is safe, if it is in the machine, the machine is not safe.