I think it does have a "play" at where the unbelieving spouse spends eternity.
That spouse is not him/herself sanctified by the waters of baptism, chrismated with the Holy Spirit or partaking of the Eucharist, but the other one is. The other is united to Christ, and he/she is united to him/her that is united to Christ. They are of one flesh with an Orthodox Christian.
Now, that does not make the unbelieving spouse an Orthodox Christian, by no means, but it does mystically unite the spouse to the church in some way by the marital union, and certainly also ensures prayers for the unbelieving spouse from the believing one, and hopefully as well from the parish community and later on their hopefully believing children. We still must say "Lord, have mercy" for him/her and cannot act like he/she is a Christian, but we cannot take likely the unity of marriage into which he/she has entered with a servant of God.
...I hope that made sense.