Should grad students also serve as lab managers?
A couple thoughts, on someone who has been now a grad student, a lab manager, and a PI:


Someone has to deal with those things, and the PI is often not the best person for that particular role. And it tends to be helpful if there's a single point of contact for things like "Have we ordered more X?". I find that "It's everyone's problem" turns into it being no one's problem.
A lab that will volunteer you for something while you're not there, the PI can't gauge your workload, etc. is not a lab I'd be enthusiastic about working in long term.
Working as a lab manager is a helpful entry on your CV, especially if you don't necessarily want to be a PI. There are often questions on this site about research careers that aren't TT faculty positions. Lab managers are one of those positions, and having experience with it etc. is a good thing.
It's also a good thing if you are interested in being a PI. Because from experience, when your lab is N = 1 people and you're just setting things up, knowing how all of that works is handy.
"...we're there to do research." And how will you do that without the equipment necessary for your work?


That being said, in an ideal world I wouldn't want a graduate student as my lab manager, because graduate students are inherently transient, and one of the values of a lab manager is as an inter-generational knowledge base. But maintaining a full-time, permanent position for essentially a staff scientist is often out of reach of even fairly well-funded labs.