You might be right that Heroku always keeps all of your dynos "spinning." If they do spin them down for the two less expensive shared cluster options, that seems OK, at least for my understanding of their service options. My customers used (for a while) the cheaper shared cluster options.

If you are paying more for the dedicated cluster options, then I agree with you that they should not be spun down.

Certainly, it is OK for them to spin down the free option dynos if they don't get HTTP requests for some reasonable time period.