Kang should not be sitting like this -- if Hurdle wants Mercer in the lineup for defense/against all LHPs, then Ramirez should be sitting more. I think there's a pretty clear cut case that Mercer is a better defensive SS than Kang, but there's an even clearer case that with everyone now healthy, Kang is a better 3B than Ramirez at the very least.
It's possible that they have some kind of conditioning algorithm stressing to have Kang sit more often now, but that seems highly unlikely, and as NAO is pointing out, Kang seems to not do as well with frequent games off.
My hope is that it's a temporary liberal resting of Kang against some bad teams, but smart money is on Hurdle misuing his best players for the rest of the season. Even today, we need to go for the kill and sweep the Rockies.