It is cool factor. Has to be. Anyone abusing a knife so intensely that the handles might break doesn't know or care enough about the proper use of a pocket knife to bother buying expensive replacement scales for their added strength.
If scales are replaced just for peace of mind because you don't like the flex, that's totally ok, but at that point you would completely change the category of knife anyway. Most knives that have handle replacement options available are ones that are designed to be fairly (or extremely in the case of a Bugout) light weight, so swapping the light, an thereby flexible, scales for metal replacements kills the point of buying that knife in the first place because all the weight saving is gone. Unless you truly love the design features of the knife because otherwise it defeats the purpose of the initial purchase.
That last part is personal opinion.
Ultimately, if getting replacements helps remove stress when using the knife, then it is worth it for that alone. But then, why not just get another knife to begin with, right?
