Well..

We know that opendir(DIR,'/tmp/mydir') won't work if it's not a directory. We also know that it won't work if the permissions do not allow it.

If we say -d '/tmp/mydir' or die('/tmp/mydir is not a directory'), then to the novice- this is like the code staring at them in the face telling them.. Yes, after this, you can rest assured, the directory is there, the string can indeed be interpreted as a path.

I think this is useful. To someone who: would call readdir() to get a list of directory entries in a directory- thinking that the list will only contain directories, excempting files, fifos, etc.

Yes, it is redundant. Yes, it's poor practice. I'm throwing it out there that it could be of use to someone getting their rationale around what's going wrong.
And yes, of course- the person does not *need* a -d check in addition to opendir.
Maybe some are too experienced to appreciate the pains and horrors of a novice learning anew? :-) Or maybe RTFM applies here. Also valid. Maybe my suggestion is too lenient and just opens up more holes than it patches, too.