I've just had one of those "duh!" moments.
I've kept away from St. Mark's Basilica because it generates even more coins that I have suplus of already. However, on beta - which I don't bother planning in any way at all* I built one just for the heck of it. And found out the hidden advantage.
It might not be worth it for the coins (the goods are always welcome) but the big thing is that even at level 1 it doubles the coins collected. So long as you are on recurring quests, this means that the most frequent quest is doubled in volume - equating to maybe 5 more quest rewards even at level 1.
I might revise my planned GB building on live worlds!
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* For a "strategy game" it is interesting to note how little actual difference there is between my beta world, started around June/July, and my live worlds, started March/April, given that I have at times deliberately messed up my beta town and never bothered with any strategic planning on it. The only other difference is that on beta, I use quite a lot of (free) diamonds. 'Strategy', eh?