I just had a look at the Hydra API documentation on Simtecs website.
Of course current applications would not benefit at all from such a system, because they would still run on the main processor. But applications wich are still maintained could be altered to use the hydra threading API for data processing. They would still run on single cpu systems with almost no performance hit, but with greatly increased performance on a multi-cpu-system. The reason why no developers used the Hydra API was surely down to the very small number of Hydra cards ever made. If however a new machine came with multiple CPUs as standard and the Hydra software (or something similar) would come as standard with new versions of the OS and be available to download for older systems, I would think some developers might start to use it.
The hydra itself made little sense once the StrongARM was out, due to the slow bus in the RiscPC, as one StrongARM was already pushing the RiscPC to the limit and five ARM710s were still slower than a single StrongARM.
A new multi-cpu-machine however would be designed with buses that could cope and come with 32-bit-compatible software (the hydra software was not even StrongARM compatible).