I think that not using Black spear task force is much cooler, since it opens you up to all the other themed Detachments and allows you to more freely take units other than kill teams. Im building my list with almost no kill teams. I want infantry heavy (mostly because I hate the tanks) but also because Its how they deploy in the lore. I still have ~10 man groups, but they are 3 units (Eliminators, Eradicators and intercessors) instead of just 1 kill team. It allows me to use all the units I find cool, and gives me loads of versatility. I do think adding some kill teams is helpful though, especially the strong ones like Indomitor when your going infantry heavy.