There are so many styles of shooting and loading, crossed with brands of brass and batch to batch variations... that it is good to have options in your tool set.
When you consider the different demands of each type of loading, it should become clear that they have many similarities but those types can benefit from slight differences in the methods and tools.
It could be said that when a traditional sizing die with expander ball was viewed, there were examples of them where runout was blamed on the tool. Rightly or wrongly, the suggestion to remove the ball and instead size back up with a mandrel was good.
Now, I mention this because not all expander balls cause problems. Sometimes the details of the die are to blame for over-constraint that did cause problems, sometimes it can even be due to the poor quality of the brass. Other times, there is nothing wrong with an expander ball set-up.
In times past, we used to take off-the-shelf expander balls, and modify them to have finer controls on the sizing process. In some stems, there is an ability to either align them or float them to prevent them from creating a problem.
When they work well, and you have fine controls, they do save an extra step and there is no reason under these circumstances not to use them. On the other hand, if they cause problems, or if the costs or troubles of getting them in fine steps to offer better controls isn't working out, then why not have the mandrel ready?
One can even come up with a sizing process that only necks down and doesn't size up at all.
I think you will have to decide for yourself how many different tools and methods you want at your disposal and try them to see which ones suite you. More good choices are better than no choices. YMMV
