They're likely feeling the heat from all the attention and work that's been going the way of Arnold and, to a lesser extent, V-Ray. ILM, longtime Photorealistic Renderman house has already done at least one major feature with Arnold (Pacific Rim) and it's not likely to be the last. They're the oldest and biggest customer, with the most Renderman expertise, besides Pixar themselves. So many other big houses are either following suit, to feel like they're "keeping up with the Joneses" or they're arriving at the same place where whatever they're doing seems easier or more accessible in Arnold.

Not necessarily faster, because it's not a renderer known for speed (Arnold). Not after adding proper texture filtering and oversampling and all sorts of features that a modern renderer needs to be successful, but it can arrive at some great looks. Personally, I don't think any other renders achieve the surface shading of Renderman precisely because of its REYES (Renders Everything You Ever Saw) micro-poly engine. It's what I like most about Mantra, the REYES-style renderer bundled with Houdini with an SL very similar to Renderman.

The real power in Renderman isn't in the dials and boxes of whatever application interface might come with it. Anyone seriously interested in taking advantage of this should consider a serious look into shader creation, Renderman SL (shading language), instead of whatever canned shaders come with it. Working in any SL forces you to really think about light and surfaces and what's really going on, not just naively cranking up or down on dials.