The feedback I've got from engineers at two UK-based floorcare manufacturers, who have conducted extensive research into carpet washing, including analysis of competitors' machines, is that the Kirby system is ineffectual at removing dirt, and does indeed leave behind a residue that attracts further soiling.
I have to ask: if, after going through the process of setting up the Kirby to work as a shampooer, then shampooing the carpet, you then have to drag out, set up and use a carpet washer - the term 'steam vac' is inaccurate, they don't use steam to clean - why not just use the carpet washer to begin with? That doesn't make any sense to me!