The problem is that slf4j honors Android's Log.isLoggable() function, which
seems correct, but unfortunately in Android it's not. The Log.isLoggable() by
defaults returns false for debug logging. DEBUG can be configured only by
gaining developer access to the phone. So, the application itself nor the user
cannot enable debug level easily. This behavior is IMHO broken horribly and
developers are simply not using that call:

http://stackoverflow.com/questions/19984838/how-to-get-slf4j-android-to-honor-logcat-logging-level

I myself have found a workaround by using the slf4j-android 1.6.1-RC1. So, I
would vote for not calling Log.isLoggable() at least for Logger.debug(); I
would however recommend to not Log.isLoggable() entirely. Thanks!