Heard the same from CX staff:

Certain ID90 tickets are only available depending on seniority.

But agree with kaka: It sounds attractive, but standby is the biggest drawback. Apparently you have to apply for leave / vacation in advance by guessing when you might get best standby availability. However, if you can't get on if the flight gets full last minute, then your leave application is still valid and you basically loose annual leave day(s) of your originally intended flight day(s). Same for return obviously. Worst case you're loosing several days of your annual leave allowance just while waiting on standby. Some are therefore only booking hotels for the core nights in the middle of the trip, but leave it flexible on first and last days. But again, it's what I've heard - I'm not from CX.