Guys, guys, you're not seeing the full big picture here. Thanks to the way bonuses work, the use of sov resource squares is extremely important for cities WITH VERY HIGH TAX RATES. Let me explain.
For instance, your city might have 5x L14 farmyards and a L12 flourmill and you have, say, Gift of Nourishment active on the city, and it's summer. The base production is 2860 food, and you have so far a bonus of +32% (24 mill, 5 spell, 3 summer).
With 0% tax rate, your actual food production is boosted another 25% up to +57%, so you get 4490 food. With the 25% default tax rate, no additional bonus granted, you get 3775 food. With 100% tax rate, a -75% penalty bringing you down to 57% of base, so you get 1630 food.
Now, enter a dolmen with 15 food somewhere nearby, and you build it up to L5, so you get 15% from there (for a measly 50 RP if you placed the city right beside it). All of a sudden, the numbers from above become:
0% tax rate, effective bonus +72%, or 4919 food ; 25% tax rate, effective bonus +47%, or 4204 food ; 100% tax rate, effective bonus -38%, or 2059 food.
In all cases, you effectively get a flat +429 food (15% of base) added onto the previous numbers REGARDLESS OF TAX RATE.
EDIT//NOTE : One food *used* at 0% tax rate translates into 4 gold at 100% tax rate.
So that 429 extra food could translate into as much as 1716 extra gold, and you "only" pay 500 gold for it (plus the RP, which would be worth another 12.5 gold plus whatever the extra materials might be worth to you if imported in book form).
A similar logic applies to all other resources too. Later on in the game, you can keep importing a small amount of books from book-production-centric smaller cities with moderate tax rates (decent RP production) to maintain the needed RPs for sov, and ramp up tax rates sky-high in your main city, allowing you to maintain an insanely strong military force (think ~50k basic ranged or ~13k heavy cavalry) without the need to constantly ship anything else in, let alone the massive amounts of either gold or food the city would otherwise need.
But yeah, the bonuses could stand to be a bit higher.
I suppose slightly pre-nerfed and adjusted to be better later on is more desirable than decent early but totally overpowered in later stages and in dire need of a readjustment by the time people have been working up to a setup with those bonuses in mind over weeks or months.
Edited by Akita - 29 Aug 2010 at 18:00