Why is there a Heart Rate Zone 0?

Pretty much every other tool I have seen and used are based on 5 zones. I know I can manually set the zones but it causes a discrepancy when I compare my data with other tools that I use, specifically in zone 2 which is important with polarized cycle training. I am sure there are plenty of users that are fine with having a zone 0 but there are probably just as many that don't want it. Can 5 or 6 zones at least be an option to select?