Apple's likely looking to unify their gui rendering path to the greatest degree possible, and moving things OpenGL is the easiest way to do this and have things still be nice and fast. They might might lose some on a $10 or $15 increase to the BOM on a laptop, but the potential savings in engineer hours might be tremendous.
There's also the chance that a desire to use OpenCL more widely for internal applications means that having a gpu that can run CUDA or the equivalent is becoming a defacto requirement for all macs.
There's also the chance that a desire to use OpenCL more widely for internal applications means that having a gpu that can run CUDA or the equivalent is becoming a defacto requirement for all macs.