What are the advantages and disadvantages of having multiple advisers?
The answer (as in case of most answers in academia.SE) is it depends, often widely from case-to-case. Unless presented with an exceptional example, I would generally view having more than two advisers as a major hindrance - the perceived benefit of having another source of ideas is negated heavily by the communications overhead between them, and the scope for misunderstandings about their role, which might cause the student to be either overloaded in multiple (possibly conflicting) areas, or languishing without any significant guidance. The rest of my answer is based on the assumption that the number of advisers is 2.


What is the defined role of the said advisers - are they equally responsible for guiding the student towards completion of their graduate studies (as I've been told is the case in some European schools/research schools), or is one of them the principal guide and the other a co-guide? The amount of time/effort invested by each would depend on their perception of how much they are actually responsible for the student's growth as a researcher.
As with most social interactions, it would help greatly if there is a good (or at least professional) working relationship between the 2 guides, as well as a healthy overlap of research areas - a new student might not be able to handle multiple research problems in completely different areas at the same time (without affecting the time to graduate, or the quality of results).
Assuming the student publishes with both of them independently, it would look good on her CV that she can produce publishable research with multiple established researchers. This could also have the side-effect of enhancing the student's research network - as a lot of papers (in CS at least) have more than 2 authors, and often collaborating on an paper could lead to more papers/research done with the same set of persons in future.