Alternates in my HS program when I coached were certainly on the team. They cheered all games, attended all practices, and learned the comp routines even though they only competed if needed.
What's funny is that I'd always have one alternate or alternate parent complain re: being an alternate at start of season because "omg my Suzy won't compete what's the point of even cheering?"
What they don't know in May but come to realize is:
1. They probably will compete. Every year I coached, every single one of my alternates competed at least once.
2. I NEED them to function. Really. My worst season coaching was one in which there was a bad stomach bug before a Homecoming performance. 4 athletes were out and very sick. I used every alternate I had for that performance. They stepped up big timetime and everything went great. If I didn't have them, I would have had to pull us from the rally entirely. Or pull a few from JV sudeline (which is less than ideal because they would be learning new material on short notice and are generally newer to cheer.)