Here at my university, we have 12-13 week-long summer break for the students excluding summer-school. It is almost as long as a semester. I guess this is roughly the case for other institutions. In fact I often observe that some grad students use this break to work in another research group usually in another country (European countries mostly, USA rarely). Even some others spend entire two-semeter academic term in another university.
My question is as follows: What would be the pros and cons for using summer-time to work in another research group away from your supervisor and/or your research topic? I guess the situation would be different for a masters and a PhD student (I am currently interested in the situation for a masters student, but general ideas covering the situation would be OK).