If I use ServiceMix when should I use Camel?

Camel is a routing and mediation engine which works great inside JBI.

So if you are already using ServiceMix then you can use Camel implement the Enterprise Integration Patterns inside JBI such as Content Based Router, routing messages between your existing JBI endpoints.

You can also reuse any of the Camel Components within JBI or expose Camel endpoints as JBI endpoints.