Oh god, so true. I did a musical that had 3 understudies. All three were hired to be in rehearsal starting the second week. That meant they were there when the scenes were staged, when the choreography was taught, etc etc. They were encouraged to do the choreography "off stage" while we rehearsed, and ask questions if they needed help. They were required to be present for all of tech, and encouraged to run lines, practice choreography, etc.
We had 3 understudy rehearsals- one to learn music (with the music director), one to review choreography (with the dance captain), and one to review any blocking questions and do a full run of the show, with tech. All 3 were unprepared for each of these rehearsals, and barely knew any aspect of their role. We didn't get through the 60 minute run through in the 3 hour rehearsal. I had a serious discussion with them about it. Within a week, 2 of the 3 understudies had to go on, and the 3rd almost had to go on (of course, she was the most prepared of the three).
Luckily after our discussion, they kicked it into gear and we got through the show. But never have I had a less-prepared group of understudies, and I think they ended up going on for about 6 performances!