Having the brackets can be misleading. The poster Narkissos pointed this out on some of his posts of years gone by.
His reasoning was that translation itself is not a word for word process. To give an example, in John 17:3 the old NWT renders ginoskosin as "taking in knowledge." The revised NWT renders it "coming to know." The Greek word has the basic meaning of "know" or "knowing." One could argue that if you were going to use brackets, it should read "[taking in] knowledge," or "[coming to] know." But someone else might just as correctly argue that the whole phrase, "coming to know," IS itself the translation into english of the greek word.
Narkissos' point was that using brackets, only sometimes, might give the impression that every other unbracketed English word has a single original language word associated with it, when, in fact, it often doesn't.
Having said that, I wish they had kept them in because some of the bracketed material was interpretational. For example, Matthew 2:2 (old NWT) says in part, ". . . For we saw his star [when we were] in the east,. . ." The revised NWT says, "For we saw his star when we were in the East." The positioning of "in the East" is ambiguous in the greek text of Matthew. It could be referring to the star. Or, it could be referring to the Magi. (But see my posts # 434 & 437 here for a completely different understanding.) The NWT takes it upon itself to decide that its readers will understand it to refer to the Magi by inserting "[when we were]." It is for occasions like this that I wish they had left them in. I would at least be cued in that there is some interpreting going on.
An opposite example can be found at 1 Thess 5:3, "Whenever it is that they are saying . . ." "It is" is not in the greek text. But it has never been bracketed. So they simply added a verb ("is") and an associated object ("it") to make it read how they wanted with no brackets to indicate the addition. (But see my post # 337 on this thread for additional.) So, even when they used the brackets it was a halfway or hit-and-miss device.