The issue is...going by the pants example...

You are wearing a pair of Cargo Pants.
The Cargo pockets are your slots.
Materia are let's say...your pocket knife and brass knuckles.
Do you see a Tailor to put them in your pockets?
Or do you just open the flap on the pockets and drop them in?

Now...
I fully agree on seeing a Tailor if you plan to add extra pockets. (Overmelding)

I honestly believe that is what everyone is worked up over.
No one has spoken out against Overmelds requiring crafters.