Could not agree more, any company I've worked at with an on-call rotation has always ensured that staff are not scheduled when they have holiday booked. The only time an employee needed to find their own cover is if something unexpected came up during their on-call period and they needed a few hours out (like an emergency visit to the doctor with a child etc).
At my current job we have an automated scheduler which uses our gcal to ensure that it never schedules if people have an AFK entry. It also schedules fairly based on how long since the person was last on-call, not putting them on on a weekend if they were on last weekend etc (we do 24hr shifts).