I guess a lot had to do with why we listen to discussions of films. A discussion about how the reviewer would have done the movie differently is different from a discussion of whether the reviewer thinks the creators succeeded -- or didn't succeed -- in doing what they may have set out to do. When we veer into the land of "this is how I would have done it" we're asking the reader or listener to evaluate the movie and our own creative abilities and imagination.
I've NEVER critiqued a movie based on how I would have done the movie differently had I made it, nor would I ever. I'm not a filmmaker. What right do I have to tell a director how I could have done it better? It would have to be an incredibly sucky, just downright ineptly made film for me to ever think or say I could do a better job.
Like I said above, if you think what we're saying isn't about the movie on screen but some movie in our head, then we're just not watching movies the same way -- or watching the same movie. Which is fine.
If Matty says The Informant should have had Philip Seymour Hoffman or someone like him in the lead role, that isn't Matty making a different movie -- that's him saying that what Matt Damon was bringing to the role, based on everything the movie is giving him and what he makes of it, wasn't working. In other words, the movie seemed set out to do one thing and the casting didn't deliver (in his opinion, of course).
If I say District 9 shouldn't have devolved into a conventional action movie, that's not because I have a problem with conventional action movies... my problem is with movies that start out so much smarter and more interestingly made, and then devolve into a conventional action movie. Again, the movie seemed to set out to do one thing and then abandoned it.
It's always about what the movie is giving us.
I also think we do a good job of using specific scenes and examples from the movies we're discussing to make our points. If anything we get complaints that we're too specific.