When you get rejected, is it ever worth giving the potential employer feedback if you think their process was a bit moody?

I ask, as having just gone through a massive swathe of them, two places have totally let themselves down imho. One was totally lax about keeping informed, and only told me I wasn't coming back for a second interview until after the dates they'd told me to keep free (as it happens I wasn't doing anything anyway, but y'know) and the other has decided that they won't be giving feedback to rejected interviewees, of which there weren't actually that many.

Normally I would just let such things slide, particularly as I've got a job somewhere else, but 1) I put a fair amount of effort into applying for both of them (and doing work experience there) and so feel like I'm owed a bit better and 2) I might have to work with and/or want to apply to them in the future, and now my resounding memory of them is icky.

So - should I/would you/do you bother giving them a piece of your troubled mind? What does one say to make the point but retain dignity?