The android slf4j binding is fine if all you want is to be able to log directly to the native log implementation, but Roboguice's Ln logger is configurable and allows, for example, logging to a file. This is an important consideration in a lot of use-cases, for example when you want to capture an application-specific or operation-specific log for diagnostic purposes.
If there's an indication that this would be used by other people, I could pretty easily provide an implementation of the bridge.