One thing that I really like about the lack of strict alignment adherence as it relates to clerics in 5th edition D&D is that it actually allows for something that was mentioned in previous editions of the Forgotten Realms, but was pretty damn hard to actually model. In this case, I’m talking about the Faithless and the False.
Questions of Faith
The Faithless are people that deny the authority of the gods (i.e. they don't think the gods have any right to control the fates of mortal beings, which may include, but may also be, different than not believing they exist). The False are people that purport to worship one god when they actually serve another. Given that, by and large, the Realms is polytheistic, this one could be tricky to explain to people. If it's okay for my paladin of Torm to say a prayer to Umberlee before a journey on the sea, how do you actually become False?
Part of the problem in previous editions was that if you were a lawful good follower of Torm, and you became increasingly tyrannical and inflexible, by the time you became lawful evil, you lost your powers, and it was obvious that you were doing wrong. Game rules actually worked against being "false."
In the 3.5 books, you could take a feat that would let you be a delusional divine caster, call Veil of Cyric. Now, from your character's standpoint, you didn't know you took that feat, but it still felt like an overly mechanical way of dealing with a crisis of faith, and people that undermine the precepts of their religion while thinking they are spreading the good word.
Because alignment has much less of a mechanical component, it's much easier to play out that follower of Torm that becomes hardened and jaded, starts doing "what needs to be done," and is actually now getting his divine power from Bane or Asmodeus. If that character realizes what they become, and they pull away, hooray, they resisted temptation or gained redemption. If they realize what they are and embrace it, and they start worshiping their new dark master, also hooray, they are at the very least no longer undermining Torm's faith by acting in his name. But all of that is much harder to do when alignment is a mechanic that slaps you in the face by taking your class abilities away from you.
I'm sure there are still DMs that yoink a cleric's spells as soon as they appear to be drifting away from their deity. And even if you don't do that, I think it makes perfect sense to plant a few less overt signs of the god's displeasure, and possibly even signs of your potential new god's favor. That said, I like the potential for a lot more subtlety and questions of faith when it comes to divine casters.
Special Bonus Round--Paladins!
I’m perfectly okay with the fact that paladins can still fall, even in light of the above. In part, I’m okay with it because of the above. Part of the mystique of paladins is that they follow a specific personal code in addition to the precepts of their faith. Its not just about failing their god, it’s about not living up to their oaths. There is a personal component to their fall.
I think it’s a lot easier to go your whole career as a paladin and not fall from grace these days, but there are still tools for you to engage the idea of a redemption arc if you want to follow that path. Which is probably how it should be. You can play the fallen paladin that seeks redemption, the Oathbreaker that becomes a nasty, cynical shell of their former self, but that's a roleplaying choice, not a GM playing “gotcha” with their vision of how you should have interpreted your tenants.