Each instrument on Mars, or any other planet, has strengths and weaknesses. Used together, you get a whole picture.
HiRISE can take visible and near visible images, in very high resolution. There's a lot that can be learned from this that is harder to learn with poorer resolution. Visible images will allow you to see real objects, and in particular, how they change with time. It is easier to relate to these images than those with other images. Furthermore, it is extremely useful in seeing exactly what is on the surface for making landing attempts, for instance.
THEMIS, for instance, is infrared. One of Infrared's strengths is to determine the size of surface particles. It can tell the difference between sand, bedrock, and boulder fields, for instance. One of the things that HiRISE has done to address THEMIS data is to confirm some of the sand/boulder fields that THEMIS has seen.
In this case, it isn't that one or the other isn't reliable, it's just that the show very different things. In fact, it is extremely common to use multiple sources to publish papers. The knowledge of the digital elevation map is actually used in the planning process for all images, and at least when I worked at HiRISE, THEMIS images were used to help plan where to shoot HiRISE, as they were the highest global resolution images of the planet. Perusing the internet, I see many references to using THEMIS and HiRISE data together, such as Possible Chloride Salt Deposits Observed in THEMIS, EXTENDING CRISM SPECTRAL COVERAGE IN GALE CRATER USING THEMIS VIS AND HIRISE., and IDENTIFICATION AND ANALYSIS OF
ROOTLESS VOLCANIC CONES ON MARS.