This has come up a few times from people who wish they could pick archetypes for the game’s joinable NPCs, like you can in Deadfire and DOS2. As a second-best option it would at least be nice if you could multiclass Amiri into mad dog, Jaethal into monster tactician, Linzi into thundercaller, Jubilost into grenadier, etc at second level. But you can’t do that in the current version of the ruleset.
You could also see this kind of thing coming in handy for, say, a monster tactician who reaches 17th level and would like to go sacred huntsman for the remaining three levels to boost the power of her animal companion. In some ways it would make more sense for MT to be a separate class anyway (could make a similar argument for viv and scion).