During discussions, especially brainstorming sessions for new features in the product, one or more of the members of the group will play the devil's advocate. This might be me or someone else, constantly bringing up things that could go wrong. This is good as it helps to make sure every possibility is being covered resulting in a robust product. but at times some of us tend to come up with unrealistic use cases. My question is, when is too much? when is it detrimental to the discussion or when is it not enough?
I think there is no hard and fast rule, except to say that you need to balance the amount of thinking vs. over-thinking required given the particular timing and resources at the stage of the project you are working on.
I can't remember the number of times when most people in the team thinks that a use case raised by someone was unrealistic but turns out to be a very valid scenario simply because there were things that people didn't know about at the time. So you don't want to dismiss ideas on the basis of it being unrealistic, but you can certainly document it and move on rather than spending too much time over-analyzing it. I think the brainstorming sessions are best used to look at variations on your ideal or most frequency paths, and odd use cases are always going to come up if you look deep enough into the analytics or research data. In essence this is about effort and the benefit you can gain out of digging too deep into odd use case, but also the risk of not contemplating and dismissing it outright.
You'll know it is detrimental to the discussion (so possibly too much) when it interrupts the main flow of the conversation, and it will be up to the facilitator to recognize it and redirect the conversation back to the main topic. What you want to do is document this branch in the conversation so you can come back to it when there is some time left. You'll know it is not enough when everyone is agreeing with each other and you get into a 'group think' situation so someone will have to pick up the role of the devil's advocate and run with it.