How should work experience affect decision to get an MLIS (or another similar specialized Master's)? The Master of Library and Information Science (MLIS) degree is widely discussed as a potentially bad investment as a graduate level degree at this time because few programs are funded, it is rather specific, and it is notoriously difficult to actually get a job in the field of librarianship with an MLIS. One of the major reasons I've seen cited for this is the number of graduates versus available positions, and lack of practical work experience among new MLIS graduates. 

However, I may have the opportunity to work in a library for over a year during my undergraduate degree. If lack of practical work experience is indeed (?) a major factor in the difficulty of the library job market post-graduation, would an MLIS be a more practical investment for me considering I could gain this experience before beginning the graduate program, thus leaving with both a degree and experience? In general, when deciding whether to apply to or enter an unfunded Master's program in a given field directly out of undergrad, should presence or lack of existing work experience in the field (or a related one) during undergrad be a major consideration?
I am curious to hear others' answers, especially those specifically about library work. 

I highly recommend working in a field before spending your time (and money) training for it. I am extremely glad that I worked straight out of college instead of trying to go directly to law school. Working in a law-adjacent area, I realized that I did not want to be a lawyer, but I enjoyed working with colleagues with Ph.D.s and chose a different path. 

If you are going to work while studying anyway, working in the library sounds like a great choice for you. If you were not planning on working, I am not sure whether it would be worthwhile to seek out that opportunity. (For U.S. students who are going to work anyway, working on campus through Federal Work Study is associated with a higher GPA versus working otherwise. However, for the students who were not otherwise going to work while studying, on average working is associated with worse academic performance. (Scott-Clayton and Minaya, 2016) (Working while studying seems to be worse academically for full-time students but not part-time students (Darolia, 2014), and the effect of working may depend on what activities the work time is substituting for (Scott-Clayton, 2011).) 

(Pedantic note: Scott-Clayton 2011 is a quasi-experimental study based on the variation in the amount of Federal Work Study funding institutions have available, which means I feel free to use the word "effect" in a causal sense. The other two references address causal concerns through modeling endogeneity and matching, which make their analyses useful but not definitive evidence about a cause-and-effect relationship.)