The SASL PLAIN authentication is most certainly useless, as one can already authenticate using the Simple Bind. However, it's still possible to issue a SASL PLAIN authentication on ApacheDS.
The difference with a Simple Bind is that the user‘s name is not DN, but a meaningful value that is stored into one of the user’s entry Attributes.
When the server receives a SASL PLAIN bind request, it will look for the first entry which uid is equal to the provided value, starting from the server searchBaseDN position in the DIT.