Are there any real live (non-academic) and public (open-source or free) examples of a semantic database like Metalog being used to solve a computing problem that traditionally had been done with relational databases?
Semantic databases work much better if only part of your data follows a schema.
If you need additional columns in a semantic database, you just add them. Even for single rows. This is hard or inefficient in a relational database.
Also clustering is much more simple with semantic or tuple databases. Most often, this means just to install the database on N servers and set a few config options.