So why be different? If less than 5% of the Western Church chooses to sing acapella- then what is the argument? It does not seem as though we have a leg to stand on in our argument. The Church of Jesus Christ DOES USE instruments in worship. That is just the fact.
But it was not always that way: Hear this selection from Nick Needham's (who, as far as I know, uses instruments in worship) book on Church history, 2000 Years of Christ's Power:
In the period 900-1100...organs began to become common features of the great western abbys and cathedral churches... At first the organ was used simply to give the right note for the monks and choir (like a tuning fork)... This period (900-1100) did not actually lead to a widespread use of instruments in ordinary parish churches and thus in normal Western Catholic worship. Even in the great abbeys and cathedrals the organ's use was limited... Some historical sources speak of an organ controversy in the 13th century, which resulted in the Catholic Church's declaring against the use of organs. Thomas Aquinas... confirms this, for Aquinas simply repeated the way that the early Church fathers had condemned all musical instruments in Christian worship: "The Church does not use musical instruments such as the harp and lyre when praising God, in case she should seem to fall back into Judaism..." In fact, it was not until after Aquinas in the 14th and 15th centuries, that the playing of musical instruments became a widespread, regular and accepted feature of ordinary Western worship.
So, the first 14-15 centuries of Christian worship were without instruments in worship. Then the Reformed churches abandoned them in the 16th century. That means that 3/4 of Christian history is acapella worship.
If I were in a church that used instruments, that figure alone would compel me to investigate the reason why.