The two types of standards are for different purposes, both valuable.
xAPI is used to pass activity information between systems, e.g. to take data from a library attendance application and import it to Moodle for use in a learning analytics model, or to export learner engagement information to an application used by academic advisors. We want to support both of these purposes.
Model specification standards are used as ways to export and import model definitions between systems. So for example, if I have developed a model and I want to share it with you, I can save the model using PMML or PFA as a standard, and you can import it to your system. (In theory you could even use the model definition on a completely different system if it had the same data types, but I don't know how feasible that would be.)
I hope this helps,