With that in mind, there is another point I would like to make:
Almighty God has pledged to preserve all Israel. That would include every tribe. This indefatigable surety required to fulfill His covenants promises and prophecy.
How anyone (let alone any Christian) could think that you could "lose" tribes with a guarantee like that is beyond me.
The loss of regional "tribes" in Israel is a fact of history. Genealogies aside, the tribal allotments gradually dwindled until they formed into two kingdoms, Israel and Judah.
They were still known as "tribes," because they were, in effect, separate sub-ethnicities, congregating around a particular patriarch. But politically, they lost significance with the emergence of the two kingdoms.
During the time of this division in Israel, the northern Kingdom of Israel became associated with idolatry, while the Southern Kingdom of Judah began to attract people from all tribes who wanted to maintain worship of the one true God. God had always intended there to be a single worship for all the tribes of Israel, and so Jerusalem was appointed to be the place for them all to gather. With the rise of apostasy in the Northern Kingdom of Israel, however, people in all tribes began to migrate permanently to Judah, and eventually became known as "Jews."
After the restoration of Israel, following the Babylonian Captivity, the tribes merged together as a single nation and a single Kingdom, comprising the "Jewish People." All tribes were resident in the nation, but the tribal divisions were virtually reduced to insignificance, politically. Certainly, the tribes were no longer known as autonomous regions--that idea had long since passed from history.
The important element in all 12 tribes was that they all, equally, inherited the promised nation. Israel was specifically promised to Abraham--not 12 tribes, but a single *nation.* The 12 tribes were merely the pathway leading to the promised nation. The significance is that all parts of Israel, namely, all 12 tribes, were equally heirs to this national inheritance.
For a time, genealogies for the 12 tribes remained important in the OT period, because as long as the Law was in play, a tribe was singled out for priestly duty, namely Levi. Furthermore, the promised Messiah had to come from Judah. And so, there were reasons that individual tribes were pointed out. But for our purpose here, the fact that all 12 tribes are referred to, equally, is so that it is known that God's promise to all 12 tribes would be fulfilled as an equal right to the inheritance of national Israel.
And indeed today, Israel consists of the Jewish People who have been drawn from all 12 tribes, without discrimination. That is the significance in mentioning them at all, I believe.
It remains important to characterize Israel as the biological posterity of Abraham, identified through Jacob, and consisting of all 12 patriarchs of the 12 tribes. It was God's plan, obviously, to include all 12 tribes for a variety of reasons. Some were set apart for various tasks, all of which contributed to the concept of a NT nation, which obviously has yet to be fulfilled. All 12 tribes contributed to the OT worship that has now been fulfilled in Christianity.