Where it is bad - dangerous maybe - is assuming two things:
Firstly that stereotypes encompass all the features of a person - from the above example - the sci-fi fan is also a father,
who likes football and works as an investment banker - all of those things are missed if we think of him solely as 'a sci-fi fan'. and we have made assumptions about him that are completely innacurate.
Secondly we assume that people we meet or see fall into the stereotype I am refering to. I meet someone at a sci-fi film and automatically assume that they are a stereotype fan - where as in fact they may simply be there taking thier girlfriend to see the film she wants. When we assume people fall into a stereotypical category we have created for them we take a risk of having judged that person wrongly and this could lead to obvious problems in a business with a client or market.
It can lead to further problems in a workplace if people are stereotyped incorrectly - colleagues can easily cross lines of offence if they 'put people in boxes', even more so if they are the wrong boxes! There is a line in an office that is banter - it may be ok for me to joke and be witty with my close colleagues but when you cross that line - when banter becomes offensive that is when you run into harrassment and bullying and the world of the stereotype is dangerously close to that.
I would proffer the idea that whilst stereotypes may seem useful they are mostly just 'easy'. Generally speaking they should be avoided. Each person that we deal with is different, everyone has their own needs, wants and things that offend them. Mine are not the same as yours even though we may look the same, even though we may share a similar upbringing. It is always best to treat people as individuals and that way you can get the most out of the relationships you form with them in the workplace or outside it. It may sometimes not be easy to remember but the best way is to examine yourself. What stereotypes do you fit into and would you want to be just seen as those features?