I suppose it depends on your school (like anything). If the teaching is great, then I'm sure it'll be useful. That said, I can count on one hand the number of times I've looked at slides under a microscope since starting clinical rotations, and each time was with a pathologist. It was nice that they went through the findings with us, but I would have read the report and come up with the same final result...basically whatever they told my team.
I will say I did do an Infectious Disease rotation which included time in the micro lab and it removed a lot of the "black box mysticism" that I think often surrounds the lab for most students and residents. It was only a couple hours in both the micro and virology lab but it helped in better understanding their standards, how much work goes into getting results back and the like, so in that sense it was helpful.