That seems like it violates the principle of least surprise.
However if I have a faulty drive that I connect to my system to recover data from it and I don't want it to experience any more writes because I'm worried further writes may break it further, I would be quite surprised if 'mount -o ro' caused the driver to write to it.
This is exactly what happens maintaining bootloaders. As time goes on, the amount of configuration to get ext4 to reliably read a possibly dirty filesystem without modifying it has skyrocketed to the point where I started putting /boot on ext2 again.