Most long time Witnesses are so indoctrinated and skilled at self deception that this will not impact them. However, even so, it is a huge and necessary step.
Potential converts and borderline Witnesses will see this. Those whose eyes are openable will see what is really going on. And this is important.
The organization has a huge churn rate for a religion. People burn out and defect much more often from JW's than they do from real religions. As a result, in established markets, they need to baptize at least 2.5% of their membership every year to break even. This covers mortality and defections.
In Canada, their baptism rate is only 1.8%, ensuring a continuing malaise in membership numbers. Publicity like dateline helps to increase the defections (even if only marginally), and reduce conversion prospects.
Just yesterday, my wife was talking to another mother from our son's school. They got onto the subject of the Catholic church molestation problems. Although both of them agreed on the culpability of the Catholic Church, the non-JW mother also said "I know a few things about your religion that aren't so good, either". This woman is very internet friendly.
So, believe me, the word about JW's is getting out there. Dateline will help, but it is only one facet of a huge PR problem facing the society.