The correct answer is (should be) you only need it when you need it, YMMV and you should test this on an individual image by image basis.
Of you can kind of jerk your knee and tell folks; it doesn’t work, don’t try the above, its all smoke and mirrors from Adobe and they are trying to fool you.
As for the final size of the process, if it works well you for, that’s not much overhead thanks to the cost of disk storage. If it doesn’t work for you, discard the DNG.
My very first HD was 45MB and cost $1000. So yeah, a 29MB going to 239MB DNG would be an issue; today it isn’t.