One reason Pompeii graffiti is valuable, is that it provides some absolute dating of Vulgar Latin features, while the modern Romance languages show only the final outcome without those exact dates.
I think you underestimate just how grateful historical linguists are for popular texts beyond literary ones. Another example is the Novgorod birchbark letters, which not only shed light on old Russian but some of their features (found in no other source) required scholars to completely revise their reconstruction of Slavic historical phonology in general.
I think you underestimate just how grateful historical linguists are for popular texts beyond literary ones. Another example is the Novgorod birchbark letters, which not only shed light on old Russian but some of their features (found in no other source) required scholars to completely revise their reconstruction of Slavic historical phonology in general.