Is low blood pressure dangerous to have?

It is relative. The most important factor to label somebody has low blood pressure is to know his baseline . If somebody is having blood pressure of say 80/50 mmhg for years and does not have any symptom at all . This should not be considered necessarily a bad idea . On the other hand if a person who had blood pressure of 140 /90 came to you with blood pressure of 80/60 , it should be cause for conmen.
Low BP. Depending on the cause of low BP it can be. If related to dehydration, bleeding or any kind of circulatory shock - it would be very ominous. Sometimes it can be normal after a full check up if everything else is ok.
Not necessarily. In general people who have low blood pressure live longer. However, if it is low for some bad reason, such as bleeding or infection, then it is not a good thing and could be dangerous. So it depends on the cause of the low blood pressure.