Even if it does not cause any performance issues, I dislike it. If the thing runs in terminal, it should be expect to be used in a script thus it would be better to make no assumptions of terminal capabilities and leave the fancy part to external tools, if one is interested. I always hate systemctl's piping to a pager by default. "Do one thing, do it well", don't try to surprise users with fanciness because at work we don't like surprises.
In practice I've had overwhelming feedback praising our use of spinners in the Heroku CLI and not a single complaint I can think of. In fact, I've had more praise for adding spinners and color than any other change we've put in over 4+ years of development.
That said, you need to be careful. Don't use a spinner if it's not a tty or TERM=dumb. Do use it in some CI environments that support it (Travis, CircleCI) That handles all the issues we've seen and everyone seems to be happy.
