Enevhar Aldarion
Oh Wise One
Presumably, the "bad thing" is that a Main Hall upgrade consumes culture and population that can never be gotten back, and they're running their city with such a tight population/culture budget that the costs the Main Hall upgrade imposes would unacceptably degrade the performance of their city.
Considering in later chapters there are researches and/or upgrades that cannot be completed unless your main hall is high enough level to hold the coins needed, that is a very poor habit to get into. Of all the buildings in the game, the main hall is the one that is most important to keep at max level for your chapter.