This is not recommended in any WordPress style guide, and I think it is a bad coding style. Beginners are using this style, maybe because it feels more like HTML …
Unfortunately, the default themes are using this style way too often, so some beginners might think it is part of a code style.
One disadvantage of this style is comment handling. Look closely at the following example and how it doesn’t do what the author might expect:
<?php echo 'Important: '; // announcement ?>
<?php echo ' enter the word '; /* start ?>
<?php echo '<b>password</b>'; /* the end */ ?>
Good luck debugging that. 🙂
Rule: Switch between PHP and HTML context only if you have to create output in of both languages. Use regular line breaks in all other cases.
Update, further thoughts: Every valid HTML file is a complete and valid PHP program. Yes, even if it does not contain a single line of actual PHP code.
If you start from HTML and add small pieces of PHP step by step … you might end up with the style we’re discussing here. That’s where refactoring comes into the game: Once everything runs as expected, rewrite the code until it is as readable as possible, easy to maintain and to extend, without repeating parts.
I guess some people are happy without this last step, and that’s why this won’t die soon.