If the wall was for the insane (as you would have to be insane not to believe in gods in a world filled with miracles and angels on a somewhat daily basis) who actually believed the gods didn't exist that would be one thing. But it's for people who dare not show enough devotion. How much devotion is insufficient is going to change from person to person. Exploring play where players are not showing enough devotion is something I'm not interested in. I also don't like the idea of every single person having to hold one deity above all others. If you worship the gods as part of your everyday life (blacksmiths say a quick thankyou or ask for strength before working on a piece of metal or gamblers ask for a bit of luck or ask Beshaba to look the other way) before they sit down to gamble. Whatever people do in their daily life will naturally cause them to venerate certain gods above others. To have to sit down and say "nope. This is the god I definitely worship more than any other" is a bit jarring and an unnecessary step IMO of character creation. I gave players a list of 10 deities and asked them to pick a patron deity OR a few deities they worship in every day life. I don't want to then make a value judgement on how those players are playing their character to say "ACtually. You didn't pray enough times to any of the gods so off to the wall you go."It fits with the idea of religion. In a world where there are real deities, why would there be a place for the dead that don't believe? The Wall of the Faithless was how the FR inserted the idea of damnation for the nonbeliever in a world where they had no place because the gods were "real." I was glad they implemented. I don't like the modern version of D&D where a player can choose to follow a philosophy and still gain power. It goes against the ideas that the polytheistic religions of D&D were built on. What is the point of even having D&D if everyone can be their own god in essence by worshipping whatever philosophy or force they feel like and gain power. If that were possible, gods would not even need to exist.
If you want a world with polytheistic religions based on ancient human ideas of polytheism, then you need constructs like the Wall of the Faithless to punish the nonbeliever.
Of course if you don't a world with religions, then run it your way. I find a world filled with gods with no punishment or penalty for nonbelief pointless and completely against everything we know of real world religions. It lacks verisimilitude and makes religion a pointless construct in the fantasy realm.