MRJ Adapter is an easier path for developers because it is easier to learn, leveraging concepts they already know, such as action listeners to handle menu items. It also shields the developer from the problems associated with compiling their code on other platforms when the APIs they need only exist on the Mac.
MRJ Adapter incorporates a wealth of knowledge and little-known tricks about Apple's virtual machines that no developer should be expected to know off-hand, especially if they come from other platforms. Things like how to bring up a folder chooser dialog, or how to open a document with the application that created it are addressed by MRJ Adapter.
MRJ Adapter implements many things the Apple APIs don't address, such as automatic handling of all-inclusive menu bars, assignment of a menu bar to be shown when no frame is visible, and more.