well similar to video, you have no use for it cos you CAN'T use it. thats cos you cant shoot past 1600 with any real confidence with the a700. 3200 at a real stretch. iso5000 on a FF camera is perfectly useable. i can shoot at 6400 now with the same confidence i shot at 1600 on a d300 which was already a stop better than the a700.
ISO 5000? I find myself having no use for it; the reduced dynamic range and loss of detail making it unacceptable (in a low light shot). We should step back from this fixation on High ISO which is as big a distraction as was the race for more megapixels. Of course there will be improvements in the current sensor techology but it will be incremental rather than giant steps until something new comes along, but I wouldn't hold your breath. More likely the DSLR is replaced by EVIL cameras.