What are pros and cons for using summer-break to work in another research group? 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).
Here are the obvious things:


Pro: the work in another group will build contacts, possibly improving your opportunities for future academic positions (PhD, postdoc, assistant professor position). 
Pro: funding opportunities, if you're in need of extra money.
Con: the work done in another group will take up time and energy, and if not related to your current work, won't advance your research (experimenting, publishing, etc.). That should probably be your priority (this is related to the comment on the question about it being strange to get 12-week breaks...)
Con: if you don't coordinate it with your supervisor, this could affect the trust in the relationship (depending on the style of the supervisor).


If you wish to pursue an academic career, in some universities respectable journal articles published are worth more than contacts when competing for a job. So don't overestimate the first "pro" I mention above. At the master's level, this is less important.

The best strategy is to coordinate the "outside" work with your supervisor for maximum benefit.