Examples..
You bought a jacket "with everything" BCD and dove in warm waters and it was fine. Now you dive in cold waters, wear thick exposure suit, thick gloves and realize that you can't reach the zipper in your integrated pockets. The pockets become useless, but you need them, so you are trying to solve the problem by adding pockets to your exposure suit. You end up customizing something anyway.
Let's say you can reach the pockets, but realize that your SMB won't fit. So you end up attaching your SMB to your BCD somehow - still customizing something.
These problems don't exist with "customizable" approach. You don't like your pockets, walk up to LDS with your BCD and say "I want bigger pockets so this and that can fit, I want velcro, because I have problem dealing with zippers with thick gloves and add bungee cords, so I won't be losing stuff". Come back in few days, pick your BCD with different pockets, and enjoy diving.
I see only benefits.