That said, if you're uncomfortable buying an RO/DI, just don't buy one! I don't think anyone is suggesting to buy one because they want to trick you into making a bad purchase.
It's more of a convenience factor to be able to produce your own RO/DI. Plus, another thing to consider is that the grocery store water is RO, but not RO/DI. That means if you properly maintain your RO/DI unit, the water you produce will be superior to the water you buy at the grocery store. And last but not least, it just sucks to carry those jugs around: water is heavy.
The only reason I suggested to watch out for rejection rates is, well, 1) it's kind of interesting and thought you might find it interesting too, and 2) You will indeed notice a difference in the depletion rates of your DI resins if you go from one to another. It might not be so much a cost factor but the inconvenience factor of having to replace that stuff every so often.
Hope this helps!