Here's an interesting point: do strong players only take Necropolises, Dungeon, Serpent Pits, and Naga Temples with their main hero ("the folder"), or do they also use weaker heroes ("scouts"), even if there's a risk of losses?

No... it's not necessarily with the main hero. Weak structures like Necropolises are usually taken by scouts in 90% of cases.
In general, it all depends on the army, the situation, the level of the scout, the established strategies, and the need for money and resources at a specific point in the game. Sometimes, even at the cost of significant losses, it's necessary to obtain a particular resource or gold for development. In such cases, taking a weak structure with a weaker hero can be more than justified.