The other responses have laid out the key considerations in deciding whether the Paris Museum Pass will work for you. I would just add that if you are considering a pass that doesn't cover the full extent of your visit to the city, you should also think about what places you want to see that are not covered by the pass or are free. Making the pass pay off will probably mean packing the covered sights into your pass days. But what if there's a non-covered or free sight located very near a pass sight? No one ever has time to see everything he wants to do in Paris. Surely the most efficient thing to do is to see those two nearby places back-to-back. But now you're wasting part of a pass day. So: How many free or non-covered sights do you want to visit? Here are a few examples (which I hope are accurate):
Free: parks, typical Parisian neighborhoods, markets, many churches (may charge for towers, crypts, etc.), Petit Palais, Memorial to the Deportation.
Not covered by Paris Museum Pass: Eiffel Tower, Seine cruise, Catacombs, Jacquemart-Andre Museum, Marmottan Museum, Museum of Naïve Art, Museum of Counterfeiting, Museum of Romantics (Musee de la Vie Romantique). There are quite a lot of others.
Since you've already bought tickets to Versailles, the calendar is a factor. You probably don't want a pass that covers your Versailles day, because there may not be a lot of time left by the time you return to Paris. Does Versailles fall in the middle of your stay in Paris? The museum pass is valid for consecutive days.
That said, it is difficult to quantify the value of being able to make multiple visits to the Louvre, or to pop into a museum just to see a few rooms.