Even full speed GPUs will be nearly worthless for bitcoin mining within weeks.
EDIT:
"Difficulty" determines the amount of computation required to create a bitcoin block. Hash rates have greatly increased lately and the difficulty automatically increases to maintain a constant block generation rate. Check out the charts here:
http://bitcoindifficulty.com/
This doesn't run anything important on the Pi. The only thing running there is a client that dispatches calculation jobs to some server with nvidia GPUs somewhere is the network.
You're right. GPU on any ARM SoC will never run full-fledge CUDA code (except Maxwell I guess), but the benefit of dynamic GPU resource allocation is still useful if we believe computing resource should be as flexible as storage in cloud environment
EDIT: "Difficulty" determines the amount of computation required to create a bitcoin block. Hash rates have greatly increased lately and the difficulty automatically increases to maintain a constant block generation rate. Check out the charts here: http://bitcoindifficulty.com/