pros: runs on slow hardware
cons: makes your game slower than that slow hardware

I wouldn''t use it unless it is absolutely necessary for the game to run.

One usage would be Vertex Shaders on a DX7 level piece of HW (ie has HW T+L). Because vertex shaders cannot run in hardware on these cards they need to be compiled and before being run the D3DUSAGE_SOFTWAREPROCESSING should be set to tell DX/GFX Card that the geometry processing will be done off the card. This has the benefit that if your app uses vertex shaders and it targetted at HW T+L (as well as higher) you dont have to run the whole app in SW - you can select only to do it in SW when needed and still use HW T+L for the rest. Be warned however that if this apprach is taken the switch to D3DUSAGE_SOFTWAREPROCESSING is apparently quite expensive - I think S1CA mentioned it being like destroying and creating a new device (and therefore should be batched accordingly - all HW T+L then SW T+L batched into 2 portions of rendering).

Neil

WHATCHA GONNA DO WHEN THE LARGEST ARMS IN THE WORLD RUN WILD ON YOU?!?!