The manual is for the average shooter who does not have the tools nor experience to go deeper into the internal workings of the gun. That is probably over 90% of S&W customers. The manual is not wrong, it just saves both the average owner and S&W a bunch of headaches.
If everything was smooth, polished and clean from the factory in a mass produced gun, curiosity as to how your gun works might be the reason to disassemble the slide components.
However, being a mass produced gun, there will be debris, sharp edges, rough surfaces, migrated lubrication, fouling, dirt from carrying or any other items of like quality in the recesses of a slide. Will they amount to producing a malfunction? Probably not, but the likelihood is increased.
In particular, the channel that the striker block rides in has been rough on every M&P I've worked on (7), and most had a sharp edge up inside the slide where the tooling cut the hole. Removing the striker assembly is one of the steps to removing the striker block. Polishing the striker block channel and the part of the striker channel that intersects it has done a great deal to improve trigger pull on each gun.
I routinely completely disassemble a slide before shooting it to polish everything, clean everything, and lube it properly. I am sure everything is assembled correctly and functioning correctly when I first go to the range. Good confidence builder.
I do a detail strip of the whole pistol for regular maintenance at about 1,000 rounds. Takes about 30 minutes. Worthwhile. The manual doesn't say to do this either, but probably a majority of their guns never reach 1,000 rounds fired.