I'm a part-time student in a department with quite a large number of students. Generally we're working on quite different topics, some related thematically, but otherwise different, so it's possible to discuss ideas and methodology without any risk of someone nicking your idea. If I was a student in an area where there was potential for closer overlap I'd probably keep things more to myself. But as it is, I find it beneficial to knock around ideas with a few close contacts.