In the hierarchy of information sources, dissertations and theses get a mixed reception. They rank higher than conference abstracts, which isn't saying much; after a few years, even a good abstract is kind of like a tooth that shows you duckbills or tyrannosaurs were present in such-and-such formation, but not much else. They get a bad rap for a number of reasons. Except in rare cases, their authors don't have vast experience writing scientific works, and the proofreeding and editorial processes are not generally as stringent. Speaking for myself, I worked on my thesis like a rat gnawing through concrete, and I wouldn't take back the accomplishment, but at the same time I could do a much, much better job today. I'd bet a lot of other people feel the same way about their graduate work. The quality of scientific review also varies, and is not considered to be of the same quality expected for a peer-reviewed paper, so questionable ideas can sneak through that would be weeded out in other publication venues. Finally, they can be difficult to nigh-impossible to find: the number of publicly available hard copies of a given dissertation or thesis is often one (1), so if the one you're looking for has not been scanned, cannot be purchased, and/or only exists in a university library hundreds or thousands of miles away, it might as well have been lost in the Library of Alexandria for all the good it's doing you. It's not hard to understand why published versions of graduate work are preferred.
If you can get access, though, a dissertation or thesis can be uniquely useful, provided you recognize the faults. They are great for detail. A peer-reviewed publication in a journal is usually on the order of twenty pages or fewer. A dissertation can be much, much longer, and contain the seeds of multiple papers plus stuff that was left out. For example, in my work on Saint Croix National Scenic Riverway geology, I've had access to many relevant graduate works through the University of Minnesota libraries, including dissertations by Clem Nelson (1949) and Bob Berg (1951) that basically reset the paleontology and stratigraphy for the St. Croix Valley. Although the two of them published several papers that incorporated parts of their graduate work (Nelson 1951, 1956; Berg 1953, 1956; Berg et al. 1956; etc.), these publications did not use everything. In particular, if you're just going by post-graduate publications, you only get some of Nelson's and Berg's stratigraphic sections.
Another thing that dissertations and theses are uniquely good for is documenting work from people who either ended up leaving the field or did not publish their results. Not publishing doesn't mean you did a terrible job; it can mean as little as your project's results didn't really have a venue. There are plenty of "Geology of the [insert name here] Quadrangle" projects that haven't been published in journals but are still valuable geologic records. There are a clutch of these at the University of Minnesota for the St. Croix Valley and nearby areas of Washington County for the late 1940s through the 1960s, and they've found their true homes as base data for Minnesota Geological Survey maps. One interesting example is Quaschnick (1959), a thesis on the geology of the Marine [on St. Croix] Quadrangle and the Falls Creek area. Although the Paleozoic rocks of Minnesota are not noted for their structural complexity, there is this one weird area in Scandia where faulting has preserved a thin sliver of St. Peter through Decorah between the expected Upper Cambrian rocks. You can visit part of this area in the Falls Creek Scientific and Natural Area, but don't expect to see much of the geology, thanks to vigorous plant growth. It was published on back in the 1920s (Peterson 1927, itself based on her 1924 thesis), but was still strange enough to warrant a revision. Unfortunately, Quaschnick's results were not formally published, so you have to go to the thesis. Another odd one: a few years ago, I noticed in geological maps of the St. Paul Park quadrangle and Washington County that an outcrop of Platteville Formation was plotted along 70th Street in Cottage Grove, but a personal inspection turned up nothing. This year, while going through Matsch (1962) on the St. Paul Park quadrangle, I found the outcrop described there; whatever it was in 1962, it's simply no longer visible today.
In the end, you still need to keep an eye open when using graduate work. Even apart from errors, often the author will make revisions if they do publish, sometimes substantial revisions. For example, when Nelson published his trilobites (1951), he named a number of new species. If you compare this publication to his dissertation, though, three of the new species have different names and seven new species mentioned in the dissertation were not named in the journal publication, instead being subsumed into other species. There are also some inconsistencies with specimen numbers. (Of course, dissertations and theses don't count for the purposes of formally naming species.)
Berg, R. R. 1951. The Franconia Formation of Minnesota and
Wisconsin. Dissertation. University of Minnesota,
Berg, R. R. 1953. Franconian trilobites from Minnesota and Wisconsin. Journal of Paleontology 27(4):553-568.
Berg, R. R. 1954. Franconia formation of Minnesota and Wisconsin. Geological Society of America Bulletin 65(9):857-881.
Berg, R. R., C. A. Nelson, and W. C. Bell. 1956. Upper Cambrian rocks in
southeast Minnesota. Pages 1–23 in R. Sloan and G. M. Schwartz,
editors. Lower Paleozoic geology of the Upper Mississippi Valley.
Geological Society of America, New York, New York. Guidebook Series
Field Trip 2.
Matsch, C. L. 1962. Pleistocene geology of the St. Paul Park and Prescott quadrangles (Minn.). Thesis. University of Minnesota, Minneapolis, Minnesota.
Nelson, C. A. 1949. Cambrian stratigraphy of the St. Croix Valley. Dissertation. University of Minnesota, Minneapolis, Minnesota.
Nelson, C. A. 1951. Cambrian trilobites from the St. Croix Valley. Journal of Paleontology 25(6):765–784.
Nelson, C. A. 1956. Upper Croixan stratigraphy, Upper Mississippi Valley. Geological Society of America Bulletin 67(2):165-183.
Peterson, E. 1924. The Cambrian geology of the lower St. Croix valley: Osceola to Stillwater. Thesis. University of Minnesota, Minneapolis, Minnesota.
Peterson, E. 1927. Block-faulting in the St. Croix Valley. Journal of Geology 35(4):368–374.
Quaschnick, R. K. 1959. The geology of the Marine Quadrangle and the Falls Creek area, Minnesota. Thesis. University of Minnesota, Minneapolis, Minnesota.