So I understand that certain words like Dignus take the ablative to signify an of relationship. Why is this? and is it only with certain adjectives of worth or merit?
Salve, Turendil. It’s mindboggling the extent to which the ablative, genitive and, to a lesser extent, the dative cases can be used to indicate different and particular relationships between words in Latin, and not just for adjectives such as “worth” or “merit”. Nor do “worth” or “merit” adjectives only take the ablative. They may or may not also take the genitive. You might look here at Allen and Greenough’s New Latin Grammar
http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0001&query=head%3D%23251
or download
Schmitz “Grammar of the Latin Language” from http://books.google.com/books?id=rIcAAAAAYAAJ&pg=RA1-PA266&lpg=RA1-PA266&dq=adjective+worth+latin+dignus&source=web&ots=qwAFqejT-s&sig=vgxHZo6bVpjLHfcwJuv6qEQzNKo#PPR1,M1
You’ll find examples of the “of” relationship with adjectives in Schmitz if you look at the Syntax sections on the use of the “Genitive” and “Ablative”. Before you even go near the ablative case, you will find loads of adjectives governing the genitive case for your “of” relationship: all present participles and substantives govern the genitive, as do adjectives of desire, knowledge, experience, remembering and their opposites, power over things, participation, abundance, fullness, and so on, and so on.