Speed is critical for scouts. Almost in every single situation, it is not worth adding anything other than a lightweight frame and speed nexus (for Mk2) to a light detector. In fact for pvp a T4 or at least T3 detector is a must. However, I think its a problem with the way that detector modules function rather than with light bots themselves.
It is in theory possible for someone to be playing with a single account and running a detector bot but in practice it makes much more sense for someone to use a detector alt since that way they can still participate in the fight using their main account.
It seems that light bots lack the head slots to be of much use in a fight. Light-ewar out runs them and assaults out tank and out damage them.
Either way there isn't really enough pvp going on to see if there is a combat role which could be filled by lights.