The purpose of a track or road pacer is to set a desired speed, provide some physical benefit (i.e. drafting), and/or enforce a pacing strategy. Technically speaking, an ultra "pacer" isn't allowed to perform any such role. Most race rules specify that a pacer is purely a trailer and cannot lead the runner, assist the runner (physically), or mule/carry for the runner. As the ultra racer, you're still the one setting the pace, and you're the one that has to decide to move forward, stop, eat, puke, and do all of the other fun things that 100 miles of running involves. The pacer is purely there for companionship and safety.
I've never raced with pacers, but there are times when I certainly wouldn't turn one away. To be perfectly honest, I've never even been to ultras at the 100k distance or below that allowed pacers .... only 100-milers.