Thought 1:
Setting addr:city to the incorporated area in which the address is located is redundant - such information can be gained through a spatial query.

Thought 1B:
Boundaries for incorporated areas can change. Such a change would then make addr:city wrong if it was set based on the old boundary.

Thought 2:
What is addr:city set to for an address outside of any incorporated area? Is it to be omitted, or set to the “postal city”? If set to the postal city, then we are mixing two types of information in the same tag, and if the location is later incorporated into a city other than the postal city, it will be wrong.

Thought 3:
If you were to order something from an online retailer and have it sent to the address in question, what city would you enter as part of the address? Even though we are using the term “postal city”, it is my understanding that most online retailers and delivery companies (e.g. UPS, FedEx) use the same cities as part of their addressing.