How good idea is it to play a Sacred Servant Paladin? None of the guides I know of ever explore the option, just as none of the caster guides seem to analyze the pros and cons of the Harrower prestige class or the options a Blight Druid has.
So, how good IS the Sacred Servant archetype for the Paladin exactly? And how good is it to take the Trade Sub-Domain? I plan to play a Paladin of Abadar, crossing to Balanced Scale of Abadar (updating Hit Dice to d8 since it's a 3.5e Prestige class and whatnot) for fluff reasons.
Dotting for later review.
Keep in mind that I haven't played a paladin since AD&D, so this is just theorycrafting. And thank you for posing such an interesting question! :)
The Bottom Line
My conclusion is that Sacred Servant looks balanced overall; but it shifts abilities from constant or duration-based to limited use. There are a few problems with that. One is that as a species, we tend to underestimate the power of small consistent things adding up over time. Another is that from personal experience, I often end up under-utilizing a limited-use class feature due to wanting to save it, just in case I might need it more later. A third is that it promotes the 15-minute adventuring day. However, it looks like it would be fun to play, and that's what really matters.
The Explanation
So. Smite evil gets cut in half, but you get a cleric domain. That's more than a fair trade -- which is good, because the divine bond looks less powerful to me. Even though the duration is technically the same, it's all limited use.
For example, at 9th level you can get +2d6 to all your channeling of positive energy for nine minutes. That's pretty awesome, but how many times would you be able to do it? Less than ten, I bet. Whereas with a weapon, for the whole nine minutes, holy would do an extra +2d6 damage to evil creatures for as many times as you could hit one, and speed would add an extra swing to every full attack. And then you could do it again later that day for another nine minutes; but the uses of lay on hands would still be gone. But then again, with Selective Channeling, it could swing the course of a battle, by adding +2d6 per party member (so maybe +8d6 per channel?).
What about that cleric domain? Travel and its subdomains are good choices because of that constant +10 base speed, of course. Other than the domain powers, you're just getting one extra spell per spell level, and you don't even get to choose it; but who could say no to fly or dimension door? And Trade is not only a very flavorful choice for your character, but one that could really put him over the top in social situations.
That just leaves Celestial Ally. Immunity to charm (from Aura of Resolve) is nice, but you already have excellent will saves, unlike other full-BAB classes. And what you get in exchange... let's see here... the ability to call inevitables once a week. Looks like an arbiter at 8th; and then at 12th, a kolyarut or zelekhut (or two arbiters, or an arbiter and a zelekhut). No material component cost, and they'll do "reasonable tasks" for free. That leaves a lot up to GM discretion; I would interpret that as a brief, non-hazardous task. However, I would also say that the better it fits their purpose, the more time and danger they'd be willing to take on without demanding payment. This seems pretty powerful to me; but once a week is a pretty big limitation.
I'm not going to into any detail on the Balanced Scale of Abadar because I think it's a poor choice for you. It's aimed at full spellcasters; the d6 is actually appropriate, to match the half BAB. Maybe if it were more like Rage Prophet, with d8 hp (since not based on barbarian), 3/4 BAB, and good fort saves, in exchange for no spellcasting advancement at 5th? But as matters stand, it's just not a good fit in my view.
The class features are balanced for a full spellcaster, who would gain two or three new spell levels over the course of the class; but a paladin would only gain one (or zero, depending on when they join). And where would the skill ranks for Disable Device come from, anyway? I'm not saying that it would be "bad" or "wrong" for you to take levels in it; just that I think it would hinder your ability to be an effective hero, and so you might have more fun sticking with paladin.
Good points there on both accounts. I also looked at Golarion-specific books, which allow me to summon some unique outsiders instead of the traditional Inevitables using Celestial Ally if I recall right. I think I'll keep Sacred Servant either way, and I will also keep the domain intact.
As for the Balanced Scale, I took a second look at it after what you said and realized that it's not a very good choice and neither does it fit the game flavor-wise. Also, I'm considering the Noble Scion (from the Paths of Prestige book) instead, despite the fact that it also seems like a rather poor choice, aside from the Leadership-related bonuses. (The DM has allowed Leadership as a feat, since we are going to play Kingmaker and it fits thematically)