Revan was never about being honourable, that was the whole point about his character and why he fell to the dark side in the first place. He pursued the Mandalorians zealously which is how he came to find Dromund Kaas and be corrupted by the Emperor in the first place.
I think as well, it's part of BioWare trying to move Revan out of the spotlight and let the characters of this era take over. It felt more like a great reference for KotOR fans (of whom the majority of TOR players likely haven't picked up), rather than as substantial as it could have been. Remember this is The Old Republic, not Knights of the Old Republic.
I agree it felt like a cop out and his 'vanishing' act at the end is likely there so they can pull him out again at some point in the future.
He joined the dark side for a reason...