The trend towards wider handlebars in XC racing, such as the 800mm bars used by Nino Schurter and Jolanda Neff, can be attributed to the increased control and stability they offer, especially when navigating technical trails at high speeds. However, it's essential to consider that wider handlebars may not always be the best choice for every rider or trail condition.
A narrower bar, around 660mm, can indeed provide better agility and quicker handling on tighter trails, as you've pointed out. This is because narrower bars reduce the frontal area, making it easier to navigate through tight spaces and negotiate sharp turns.
The real benefit of going wide lies in the increased leverage and control, which can help maintain stability during high-speed descents and navigate rough terrain. However, there is a trade-off between agility and stability, and finding the right balance depends on the rider's preferences, riding style, and the specific trails they frequent.
Ultimately, the choice between wide and narrow handlebars is a matter of personal preference and should be based on a careful assessment of the rider's needs and the demands of their local trails. It's not a question of being "stuck in the dark ages" but rather finding the optimal setup for a given situation.