So again we are talking rich apps. Itunes, office, development apps, etc definately will not benefit?
The purpose of a MBP is to provide the power for hungry applications. So from that sense the MBP does it's job with Snow Leopard.
Grand Central will benefit Applications that have been written to take advantage of it. This is basically an easier way of parallelising tasks within the application - so all desktop apps may reap rewards.
OpenCL is basically using the GPU which is excellent in a specific maths context but not for normal parallel issues such as attempting to do multiple generic tasks at the same time.
Where the discrete GPU with does make a difference is that it reduces the load off the memory bus connecting the CPU and memory (such as having an additional monitor plugged in). This means a quicker machine for normal applications.
Window managers now also make use of a lot of effects - these are better handled by a discrete GPU for the points reasoned above.
VMs such as windows, video encoding etc will benefit from a discrete GPU.
However you're right. Most machines now are very capable of doing the mundane tasks such as email, web, watching DVDs and word processing. The industry is moving to make software more efficient (greener) and so slower hardware can be used to extend battery life - Snow Leopard is Apples response to this trend.