It is well-known that the more expressive a representation language is, the harder it is to perform inference in the language.  Clearly, the corollary is that we should use less expressive representations.  So why am I trying to sell you a language that combines relational logic on the one hand, probabilities on the other?   Well, complexity can often be misleading.  Polynomial in the size of a representation is great, but not if the size is itself exponential.  Specifically, many tasks are linear time in an atomic representation --- we simply enumerate the worlds.  But the number of worlds is huge. Thus, more expressive languages are not necessarily harder.  Furthermore, expressive languages often contain structure that allows us to perform tasks much more efficiently.  So, by picking the “right” type of expressive power, we can often do inference much more efficiently.