Yes it is. Developers aren't perfect and stuff falls through the cracks all the time. Sometimes the player base needs to actual get their hands on stuff to fully test and make sure its ready for release as well.

A PTR doesn't have to spoil anything, it can be limited to one zone or a select number of fights. Hell they can be old zones and fights as long as you can test the classes and see how the skill and job balance is working.

It would save a lot of manpower(no need to hire anyone to test really, players do it for free) and it leads to a better game overall.