Conditioning on a collider is the correct thing to do when:

1. The only way to collect data to answer your question is by sampling based on the collider. (For example, the collider may be admission to a certain hospital, and the only way to sample enough cases of a rare disease is to recruit based on hospital admission). This is what is happening in the monty hall problem; the data is collected with conditioning on the collider being built in

2. Conditioning on the collider is the only way to block an open backdoor path, and the path opened by conditioning on the collider may be blocked by conditioning on another variable.

In this situation, the "true" probability $\theta$ of choosing a door with a car is 1/3. If your goal was to estimate $\theta$ , you would want to not condition on the collider, which would result in you choosing a car with a door 1/3 of the time.

However, if your goal is to get a car (and not to accurately estimate $\theta$) your priorities change. So, you will make decisions to introduce upward bias into the number of cars you will observe receiving.

In an analysis, you want to condition on a collider when:

1. The only way to collect data to answer your question is by sampling based on the collider. (For example, the collider may be admission to a certain hospital, and the only way to sample enough cases of a rare disease is to recruit based on hospital admission).

2. Conditioning on the collider is the only way to block an open backdoor path, and the path opened by conditioning on the collider may be blocked by conditioning on another variable.