For me, it makes no sense that Leon makes it to the game and Ein doesn't... Leon doesn't participate in the story and nobody plays as him while Ein has a big fanbase (myself included) in the four games he's appeared.
Seeing how easily offended you still were after Game Over - of all people - tried to explain things to you nicely before, I'll start by saying I'm not here to talk trash, just truth. And if you're offended by this, I am truly sorry for speaking out. With that out of the way...
Leon would serve a purpose in the story here if they dug deeper than him only fighting for his dead lover. Remember, he has a history with Bayman back when they were trained in Command Sambo. Chances are he may have taught him everything he knew. If Bayman gets more backstory than expected in DOA5, it could be the most logical reason he still serves a purpose there. For all we know, Team Ninja probably had writers block at the time with some characters in DOA4 during Itagaki's time, and is picking a time like now to give characters like Leon (who had connections to more involving characters) more insight after the feedback on DOAD's admittedly biased storyline. There was also another question that remained unanswered: Why did Zack remind him of someone in DOA2?
Ein, at this point of the DOA storyline, is unfortunately an amnesiac persona. When Hayate gained his memories, his impact with the story has been little to none because technically, he doesn't exist anymore. Until there's a plot device that suddenly tells us Project Epsilon was successful all along, his only purpose would be in the other single player modes and versus modes as a bonus character (if not eventual DLC). And to make Offbeat Ninja happy. But that's all.