Consider questions like What are the disadvantages of first-past-the-post electoral systems? This question only asks about the disadvantages, not the advantages. The problem is that someone will then start a questions - "What are the advantages..." and then we'll have the answers split over two questions. I think having these questions together - asking about advantages and disadvantages is better.
Yes, and it's not just to avoid mostly-duplicate questions. Pretty much every real-life issue has both advantages and disadvantges, and often the issue looks very different when the pros and cons are contrasted,
For example, suppose my question was:
What are the advantages of this new, "In Vitro" youth treatment?
I might get answers like, "It really makes your skin shine!" and "You'll never age another day!"
But it might not get the truthful answer, "They completely enclose your body in glass crystal, preserving you like a moth!" - because it's not an "advantage". But it is important information that put the advantages in perspective.
Another issue is Confirmation bias. Personally, I don't think we should allow questions that encourage such bias by only focusing on one side of an issue. (At least when they ask for lists.)
I completely agree. One pattern I have seen on other SE network sites is that questions that focus on one side of an advantages/disadvantages questions (albeit usually focusing on the disadvantages site) usually get closed as "not constructive". When they are split, that usually brings about more argument about someone's position. When they are grouped together, each answer would have both sides of the argument, balancing out the ideas, and keeping the question under control, as well as giving a full answer to the question that will be useful in the future to people who visit Politics.SE.
I partially agree ;)
I would much more like to have people be able to also post questions about only one side if they do reflect the other side in their question. As an example you could ask about the advantages of dictatorship after having illustrated in your question which disadvantages you consider relevant.
While this would allow for questions to be splitted I believe it is much closer to what an average reader wants from SE