The Shaper Tool is Adobe's attempt to remain relevant. It's really not necessary. Like many things it may make an operation here or there quicker. But there's nothing the Shaper Tool does which can not be done otherwise. It all comes down to working style.
There are other features in Illustrator which are superfluous and only beneficial for a specific workflow or artist. Among these are the Bounding Box and Live Paint Bucket - neither of these features offer anything unique, but they can make some operations faster or easier for some users. The same can be said about the Shaper tool. If you find it useful, then that's wonderful. Use it every time you feel you need it. There's no harm in that. But it's not "mandatory" to use it by any means.
The primary issue with tools/features such as this, is that if you get used to using them, you "lock" yourself into Adobe's app. You can construct artwork and edit it without these tools. And that construction will convert to methods in other software if you understand the underlying construction. By using the "shortcut tools" - as I like to call them - it's easy to get accustomed to grabbing that tool and doing what it does. Then when that tool is no longer available, it can be a nightmare trying to accomplish the same tasks. It's fine to us them, but you really should understand what specifically they are doing and how you can accomplish the same thing without using them.
The Shape Builder Tool is a bit different.
While the Shape Builder Tool essentially replicates Pathfinder operations, there are times where Pathfinder simply can not complete the joining/dividing of some shapes due to how the shapes themselves overlap and the desired end result. You can't use Pathfinder to combine half an overlap. It just won't do that. But the Shape Builder tool will do it easily.
If you were to disregard the Shape Builder Tool, replicating some of its operations can be very time consuming and intricate. So, in this respect the Shape Builder Tool does complete some unique operations that can not be easily replicated otherwise.
I, personally, have never needed the Shaper tool beyond the 15 minutes of playing with it when it was first introduced. However, I'll use the Shape Builder tool frequently in the creation of a piece.
Ultimately it makes almost no difference what other users think of a tool or feature. If you like it, you use it, and you see benefit in it, then that's why it's there.
There's no "right and wrong" or "better and worse" when it comes to which tool or feature you utilize really. As long as you get the point you what to get to, then however you get there is the correct way.