I did one when I was doing my MBA. It was an additionaly opportunity to network, but it wasn't very easy to do it, given that there were too many people there, and meetings were short.
However, in some cases, it was the only opportunity to network with a few companies that didn't come to campus. Meaning that it can be more useful for T2 or boutiques than for MBB. It also gave me the opportunity to network with some corporates, and for those it was a must. I don't think I would be in their radar otherwise.
But you don't have to think about this only through the perspective of someone who wants to be hired, but also the perspective of someone who wants to choose the best place to work. And to me it provided very valuable insights on the different companies - which were the ones where I felt I would better fit in vs. those I felt were backup options.
This also provided useful input for fit interview questions. I better understood their culture and people and knew how to press their buttons in order to get an offer (from the “fit” point of view, but also learned a bit more about their interview process).
Finally, it gave me an initial understanding of the city I was applying to. I decided otherwise in the end, but if I decided to move there for the summer it would have been very useful.
Is it worthwhile? If you really want that specific location, I would do it without looking back. If you don't care about that location, then it is a matter of financial availability vs. time availability vs. need to maximize your chances (on time availability: it may be more useful to spend 3 days practicing case interviews, if you think you have good chances of getting an interview).