... That is the question.
The answer? "It depends". There are situations where a shim pad can be a huge help and is actually preferable to flocking adjustments; there are some times when shim pads are a necessity, and there are times when shim pads will be of no help whatsoever.
Let's start with the "no help whatsoever" and work toward the positive. There is no shim pad in the world that will correct a truly bad fit. They can't compensate for a saddle that's too narrow or the wrong shape tree, and they can't compensate if the panel configuration is totally wrong for your horse. There are plenty of pads available that will claim to end your saddle fitting troubles forever, to make any saddle fit your horse, to improve your horse's performance and your riding ... But I've yet to find one that's of any help if the saddle really and truly does not fit.
As for them being a necessity: if you have a foam panelled saddle that needs some adjustment, shim pads are the only way to go. You can put shims between the panel and the tree, but they can move or slip out, while shims in the pad will stay put. Shim pads can also be a necessity when you have a horse with a long-standing and dramatic asymmetry for whom flocking corrections just aren't enough to "fill in the gaps".
Shim pads are a huge help when you're dealing with a horse whose back will be changing rapidly, such as an upper-level horse coming back from a lay up, or a young horse just getting into serious training. When you have a lot of rapid changes, particularly muscle growth, adjusting the flock repeatedly really isn't your best option. First, it's going to be very, very expensive to have the fitter out every 3 or 4 weeks to make adjustments (and taking flock out is tough to do without leaving divots). Second, repeated adjustments (especially removing flock) really aren't good for the integrity of the flock, and your saddle will probably wind up needing a strip flock (all old flock removed and replaced with new) much sooner than it otherwise would. Finally, shims allow you to make immediate adjustments to allow the muscles room to grow, which will prevent asymmetries from developing.
While shim pads aren't usually a long-term answer, they are a good answer for some issues, and are a valuable addition to a saddle fitter's tool box.