Actually I think that the advantages of networked communicating vehicles will outweigh the possible disadvantages related to security. Vehicles that communicate are much safer and more effective.

Not networking vehicles because they might be hacked into a botnet is a little bit like not networking personal computers for the same reason. We could have just decided to not have an internet. Or we could have decided that we wouldn't allow people to print flyers because they might organize a revolution.

We will definitely want communications security though.