A) There is no defense strong enough to stop a determined attacker.
B) Being attacked successfully doesn't hurt you at all unless you're plundered, which leads to...
C) Being plundered is a minor annoyance at worst, unless your city is woefully underdeveloped.
D) Putting up a City Shield is one of the most expensive Tavern boosts and so can't be used indefinitely, and while it's up...
E) You can't complete any Daily Challenges involving Tavern boosts without using Diamonds.
From these points you can see that being attacked is no big deal, even if the attacker is successful, and having the City Shield isn't a permanent defense and can actually backfire on the user with Daily Challenges. At best, it can be used on a temporary basis when in a particularly nasty hood shuffle.
As far as taking away from the game and/or not being in the spirit of the game, those are purely personal opinions that are not widely shared.