I would guesstimate that 75%+ of the "value" of my PhD came from casually interacting with smart people (classmates, postdocs, profs) working on related topics.

I would guesstimate that 75%+ of the "value" of my PhD came from casually interacting with smart people (classmates, postdocs, profs) working on related topics. These same people go to conferences, but they are busy there--they have talks to see, posters to visit, and friends/collaborators/funders to catch up with. In contrast, when you're "around", it's possible to muse and mull things over with the grad student waiting for her turn in the darkroom or the postdoc brewing coffee. "Hey, did you see this paper?" or "How do we know that <X>, anyway?" I'd worry that a part-time student would miss out on a lot of these opportunities; by definition, they're around only part-time and most of that time is "business time."