If a summary line is a shorter version of the main text, that's a good
thing. It's like a good title for a news article or a research paper: it
restates the main text briefly, and there is real value in that.

I disagree with your perception, but I guess we are getting into the
gray area where personal preferences mean more than anything else.

I suppose it could be called a personal preference, in that I prefer to follow the common practice of summarizing the commit message in its first line. This standard for Git, where the summary line is commonly used to good effect. We old-timers know about predecessor version-control systems like RCS and CVS where other commit-message conventions might have made sense too. But those days are gone, and good riddance if you ask me.

Today the primary use of a summary line is to give readers a quick way to see whether a change is worth looking into in more detail, and authors of patches should keep this in mind.