What people get wrong is that they tend to think they need to check defined() because of blank lines.

Really? I've never encountered people thinking that. Why do you think people think this way? Have you ever seen code where people do:

    while ($line = <>) {
        ...
    }
[download]

and expect it to read in just one paragraph, and not the entire file?

Having said that, assuming you are right, it would be better to teach them the proper meaning of defined, instead of discouraging them not to used 'defined'.

Abigail