I think different types of practice will help different racers in various ways.
A lot of track time will help in massive amounts to new racers and racers who have been away from the sport for a long time (like myself). Just getting a feel for what a car does with a simple, baseline setup can go a VERY long way in improving a racer's ability and confidence.
Practice and testing will help more seasoned drivers and those looking to extract a tenth of a second from each lap. They will structure their practice time much differently than I would, if given a full day of track time. I'm looking for time improvements, too, but my steps will be in much bigger chunks, because I'm still trying to run a clean 5 or 8 minutes in a row. Wasting expensive tires and batteries on raw track time doesn't make sense to a lot of new drivers, as the bulk of their money is spent on at least getting good equipment. I don't think that necessarily using your best stuff for practice in these instances is needed for everyone. I'm certainly not going to use a new body, new tires and the best packs I have to go out and pound laps in on a practice day. When I will be in the position to scrape off tenths and hundreths (few of us are), then the race gear will be there for practice days.
Just another viewpoint from someone who only races maybe once a month, and needs tracktime for more than just tweaking a setup.