I switch harnesses regularly depending on the model providers, and I only use skills which are universally usable in any model and harness combination. I don't want to get tied to a particular model or harness, and skills provide me the freedom to do so.

I can include any custom programs I need in a skill, and the SKILL.md file is just a doc of how to use those. It helps me save tokens and ensures determinism with repeated workflows since redoing a task is just calling a skill's program.

I also load only skills that are necessary for a project. I don't want to pollute the context window with unneeded skills.