They appear for both comic relief and a believable threat and most importantly, they aren't overused. In short, in every conceivable way except appearing via cathode rays, this is exactly a third Doctor and Jo television adventure from 1973 that reminds you exactly what it was about Doctor Who that attracted you in the first place. Terrance is so good at this that Catastrophea really ought to be held up as the standard for other Past Doctor Books writers to aspire to. The plot is simple, without being simplistic. There's enough going on to keep the book moving for its entire length.
Events move along believably and there's no idiot-plotting required.
Padding simply isn't an option here. The best thing is, there's an attempt at complexity, which, surprisingly, works. The moral dilemma that's set up belies the childish simplicity of The Eight Doctors and even the drug sections make far more sense than they did in his previous work. I only have a few niggles and they're relatively minor, hardly spoiling my enjoyment at all. The scene where Jo and the Doctor are first attacked by Rekar could have worked just as well without Jo pointing out that Rekar intended to rape her.