TL/DR If you need the features of 'professional' gear it's worth the expenditure.
After wading through the question body I think that your actual question is:
'Why spend the money on 'professional' gear. If you're not going to charge extra for having professional gear'.
There's a saying along the lines of (if anyone can find the actual saying, please comment or edit).
'The gear you have is fine. Till it isn't.'
Lets explain this in more detail. You're shooting a wedding in a church with an entry level camera. You find that the inbuilt flash is awful. It's very strong and you can't direct it. Thus you buy an off camera flash. You can change the power and direction.
The next wedding you're in you're not allowed to use flash and it's dimly lit, so you have to crank the ISO up, but the noise is terrible. To avoid this in the future you buy a body with better low light capability. You choose to go full frame. But now your old lenses for you entry level camera don't work so you have to buy some new lenses.
The next wedding. Still allowed to use flash, but you're not allowed to get close to the bride and groom. You're at the back of the church. So you buy a zoom lens.
Next wedding you find you're outside and it's raining. You really need equipment with weather sealing. There's motion blur, you need more light so get lens with a wider aperture. Your camera breaks before the shoot. You need a second one whilst it's in repair etc etc etc.
In time you build up your gear as you need it. In the end I expect for wedding photography you will find you probably need 3 FF bodies (2 to use, 1 as a backup), mid range zoom, long range zoom, flashes and anything else you've accumulated such as primes, UWA.
Wedding photography however I would strongly advise not taking this approach if something goes wrong and you muck up a brides big day (and her mothers!) because you were inadequately prepared and all the photos are grainy/motion blurred. On your head be it.