A player is judged fforemostly on their bowling, batting or a combo of these skills.
If all things are equal, then you look what the team needs more. You could argue that Kallis' fielding is great for making the slips tight, or Imran being an ATG captain for an ATG team can get the most out of their performance. I think the latter matters more.
Despite how the intial question was raised, I do believe fielding is actually a secondary skill as well. And not only in the slips in tests, but in all forms of cricket these days.
So let's take Imran and Kallis out of it.
As a coach, what would you want more on your team, a great captain or a great cordon? I totally agree with TJB, outside of the crazy extremes the impact of captaincy is minimal. Lloyd was supposedly a much better captain than Viv, but results wise, there wasn't a drop off.
Getting the most out of your team is basically maintaining your own performance so you can set an example and lead from the front with credibility. And not being a dick so that when you speak, they listen. The rest you pick up with experience.
Additionally, we somewhat still by default give it to the best player. Even when we are picking ATG teams, isn't Bradman always the captain, almost as is by default.
And I know this one is likely just me, and possibly just a few others. But when picking teams, I factor in who the slip fielders would be and how best to get a good balance. While for captain I just choose the best one from the players already selected.
Plus let's be honest, we all armchair captain at home while watching matches and think we can do a better job, set a better field, rotate the bowlers better, while I can for sure say I couldn't catch a cold in the cordon. And slip fielding is up there with opening and wrist spin as among the toughest disciplines in cricket. The great ones just make it look easy.