Validation - Custom callback not behaving as expected?

[eluser]Unknown[/eluser]
I just experienced the same problem. One my fields is not required but I still want to validate it through a custom callback function.

Validation is not carried out on an empty input unless the “required” switch is used.

Is this a bug that should be reported? Or is this supposed to be normal behavior? I'd like to have someone's input on this.