The docs are not really clear, but seem to imply that, to bake, running the baker and the endorser are mandatory (or at least desirable). In a number of places the documentation states that running the accuser is "optional". Why?
I'm not sure where in the documentation it says that but my guess for the rationale is that, while every participant in baking and endorsing increases the security of the network, the marginal gain for the network of someone running an extra accuser once a few are already running is low. However, the marginal gain for the baker isn't low, since accusations can come with high rewards.
Your baker and your endorser are working for you : they create blocks and operations with your signature, and you get rewards for that. The accuser is different: it monitors the network, detects double-baking or double-endorsing, and sends an anonymous operation on the network, that can be inserted by any baker, not only yours. So, even if you don't run an accuser, it is enough that there is one accuser on the network, ran by somebody else, that detects the double-baking and generates the operation, your baker will still be able to use it, and you will receive the rewards.