Unless the prespray is specifically designed to rinsed with plain water (which most are not), then plain water is a less effective than water with a rinse-aid surfactant, at a minimum.
It comes down to the fundamentals of surfactant chemistry. The better a surfactant dissolves oils, the more hydrophobic it is. That's not to say that it is insoluble in water, but less soluble than desired for using water as a sole means to rinse extract with. You can compensate for the less solubility by using more water. Not a problem if you are washing dishes at home. In that situation we practically flood the dish to remove all the "sticky" surfactant and "soil".
Carpet cleaning is a different story. In this situation, we'd like to minimize water, if for no other reason than to make the job go faster, with less rinse strokes. The other benefits are that you can use less water, which means less to extract, less to over-wet, less to have to dry. Ask yourself what you are now doing to compensate for your "chemical savings". I'll tell you. You are ramping up jet sizes, you are putting down way too much water, and then you have to use more vacuum strokes or a larger vacuum setup to compensate for all that water.
When figuring your overall chemical costs, ALWAYS, ALWAYS, ALWAYS take into account what that chemical is doing to your bottom line. If it provides for even a 10% decrease in labor expenditure, then you will find that you have a huge budget for chemical, or a modest one with more profit and less time/fatigue to complete the task."
