I certainly don't agree with some of the renderings in the NWT but in literal and dynamic equivalent translations, the bracketing and italcizing serve the important purpose of alerting the reader that an interpolation has been made. The translator has little choice when the alternative is an awkard or incomplete construction in the target language.
We may not agree with what's in the brackets, but in and of itself, it's an honest practice and not all literal translations do you this courtesy.
All sorts of unnecesary shenanigans occur in paraphrased translations and there are no brackets to alert the reader. The Jerusalem Bible for example, opens 1 Corinthians 7:2 with the words, "But since sex is always a danger" a rendering contrived out of thin air. (The NJB corrects this embarassment.)
With some translations like The Living Bible and The Message, I think half the text would have to be bracketed