It is not the only title. The reason Mother of God is effective and useful is firstly because it is true, and secondly because people who deny it do so on basis that obscure the salvation of mankind.
Mary is the Mother of the "Kyrios," the Lord God and Savior. (Lk 1:43)
I guess its proper to enumerate a summation of points here.
1. Jesus is unambiguously God.
2. Mary is the Mother of Jesus.
3. Mary is the Mother of God.
(IS 9:6-7, Jn 1:1, Jn 1:14, Lk 1:43, Lk 1:32, I have plenty more)
To deny the third point is to deny a reality of the first point. Thus I’d quote (1 John 2:22)
The Title Mother of God does not imply diminished Godhead. It does imply the truth that Jesus is God. The truth is that Mary bore the Second Person of the Trinity in her womb. She shared Her humanity with Him, and gave birth to Him. She also raised Him.
That Makes her the Mother of God. The Humanity and Divinity must be communicable realities in one person that way. Its not ambiguous, it is not heretical, it is wholly Orthodox (I’ll give a nod to the Eastern Churches for whom we can thank for the struggles to retain Christianity).
If *mother of God* were so effective and useful, then why didn’t the Holy Spirit use it?
Why did HE refer to her as *mother of Jesus*?