The way I think of it is that being divine has power in it self and the concepts like justice is a different source of power but is compatible in either additive or multiplicative way. Hence portfolio's can be fought for.
Using that, a paladin can follow the concept and use it as a catalyst to channel the power of the god who is governing it at the time. So technically the oath can be to the concept and not necessarily the god. The God gets the benefits regardless.
Asking to choose a deity is really the same as alignment to a certain degree. I'd rather not be a slave to anything good or bad. I'm spiritual rather than religious and unless you think for story reasons you will die permanently, be sent to that gloom city, be there long enough to be harassed by devil contracts and curious celestials, then put on trial; I don't think it matters to pick a deity unless you are a cleric specifically. Hell, even Oath of vengeance Paladin's straight up have nothing to do with deities.
It's pointless. Not as silly as being atheist or agnostic in dnd but still kind of silly. I mean really at one point Ao forced gods to be on faerun. Priest probably had heart attacks of joy. Even the evil ones lol.
That's how I think of it. Overall I'm against picking a deity. If an angel, god, or celestial wants to help me then sure whatever, but I'm not kissing anyones ass, feet, or whatever brown nose technique a mo fo comes up with. At that point I'd pick a fighter instead of a paladin, for example. Also why i'll never play warlock or cleric.
