You do know that that the app that goes with the NWT has the Byington, American Standard and King James parallel translations don't you? Many JWs use it and find it very useful when trying to get an accurate meaning for some difficult to understand scriptures. It has even been recommended to do this when doing personal study. JWs have never turned their noses up at other translations. And they had used the King James Version for a long time....
Also: "In addition to the New World Translation, the Society has printed on its own presses or has commissioned the printing of the following Bible translations: the American Standard Version, The Bible in Living English, The Emphatic Diaglott, Holman’s Linear Parallel Edition, the King James Version (including the Bible Students Edition), and The New Testament Newly Translated and Critically Emphasized, Second Edition" -- WT 2009 5/1 p.25
".....Theocratic Ministry School library at the Kingdom Hall provides means for research in publications that might not otherwise be available. Thus, it should be well equipped with a variety of Bible translations......" --KM 4/97 Question Box
The biggest reason for JWs using the NWT is that it has Gods name in it: All scriptures inspired and beneficial says on p. 327: "...... a number of modern Bible translations have been published that have done much to help lovers of God’s Word to get to the sense of the original writings quickly. However, many translations have eliminated the use of the divine name from the sacred record. On the other hand, the New World Translation dignifies and honors the worthy name of the Most High God by restoring it to its rightful place in the text."