Hi franchute,
good question. First, I think when you can predict with a high degree of probability what your opponent will be taking (not all readers/10T have any armor) it definitally changes what models make sense.
But, in your case, it sounds like you took joss because of interactions within your crew, and he happened to also perform really well against armor. I think that's the key, joss, like most models, shines when you take him in crews where there is some multiplicative effects on his power (in this case mobility), and doubly when you happen to hit an opponent he has a strength against.
Taking him in a crew that has not a lot of interaction with him because you know your opponent has armor is probably fine, but I'd rather assume most of the time my opponent has a few different viable options and I won't know 100% ahead of time their choice.
I do think, depending on the faction, you should have at least one tool that, for wahever reason is good against armor, but there are a lot of options for that.
IMO, joss is not the captain - he's a situationally very powerful model, I think the reason I think of him as a trap is because people see him shine in Ramos and think that means he'll perform close to the same level in other crews (like Colette or sandeep for example).