Saw Subbu speak at RestFest on Saturday. His talk seemed to promote REST as a set of constraints which produces a set of benefits. He argues that choosing a subset of constraints to achieve a subset of benefits is perfectly valid.

I would add that you must first have a deep understanding of the constraints and their associated benefits in order to make those types of informed decisions about your system's architecture. In my experience, the constraints are so orthogonal that a great number of the benefits of a system created using the REST architectural style only become evident in the overlap as the constraints empower each other.

Combining these constraints can elicit new benefits.