Why should I be a Jewish Christian / Messianic Jewish Believer?
You shouldn't. Be a disciple that takes the entire Bible seriously, and learn what scripture applies to you. As you draw near to Him, He will draw near to you, and as your relationship grows, He will guide you to the people He wants you to congregate with. It's not about labels; it's about obedience. There are too many people in the Hebrew Roots and Messianic Jewish world that do it for all the wrong reasons. You do it for the right ones and you won't have to worry about the rest.