Is there any point in finishing assembling the dragon banner in the main quest?

[Oedipus's Stepdad]

It causes every faction to declare war on you/your allied faction that never ends until you conquer them all, essentially ending trade and making it pure chaotic total war.

[Duuvian]

<makes Tigtone face>

[Kernest]

Well...

That's the quest.

If you don't like it, don't assemble it. Personally I just play Sandbox.

But even there, the ultimate goal of the game is to take over the world, nothing else really to do in the game.

[LostScout]

Once you assemble the banner you can carry it in battle. It looks kind of cool and gives a bonus against horseman to your formation. There are better banners but it isn't useless. As long as you don't give it to a king or form your own kingdom until the banner quest expires there is no reason not to go ahead and assemble it.

[Kinneas]

Makes no sense:
You gather the banner, support an empire faction...
- The leader you support then dies or old age or other.
- The leader is incompetent and gets wiped out.
- The leader squanders most of the map territories and then gets wiped out.

You UNIFY the Empire but:
- Other empire factions still attack you and do not align under YOUR empire.
- The 'CONSPIRACY' you STOP then kicks in and now EVERY faction in the game is against you and wipe you out.

WTF is wrong with your game!?

When you unify the empire all empire factions should unite under your faction and will then help you fight the others.
---
The southern empire which I chose to support had the biggest map advantage when I finally assembled the banner.

I stopped the conspiracy in its tracks.

We secured the needed 13 territories but the conspiracy happens anyway!

The other empire factions did not unite.
The southern empire has now been reduced to 1 (ONE) city and every attacker is a 400, 600, 800, 1200+ army.

Hundreds of hours playing and THIS is what it's all about?
BS.

[MasterShake]

It's a quest. With known steps and consequences. The point in finishing dragon banner is to finish the quest, and, yes, in order to do that you must have answers to the consequences mentioned (war declarations and total war).
If you don't want to do that - there's no point in assembling the dragon banner and challenging the quest. (And the game lets you get away with it actually!) In game terms, the banner is a symbol of world dominance, by lore, assembling it means - ultimately challenging the world factions, be it imperials or "barbarians", depend on your choice. What exactly do you expect after doing so?

Simplicity itself, no? Why people keep asking.