Hello Archon,
You're plain right, it grants no advantage at all except being able to add coalition standards in your embassy to allow more players to join your coalition, and otherwise gives only drawbacks, as some of your daily quests become more difficult (you're asked to raid or besiege cities of impossibly high and rare levels).
It once gave drachmas, so it was worth levelling up, but since this buying option was added, it became obviously absurd, so we don't even have this reward anymore.
Now this feature, as many others, probably came from players' suggestions. Another good reason why to think twice before posting such things... :)