How crazy is "sudo qemu -hda /dev/sda"(mounting host's sda as client's hda)?

I would say it is incredibly dangerous.

You are right in your supposition that it would likely damage your /dev/sda.

Two systems accessing the same block device, each with their own cache and buffers, will definitely mean that each has a different idea of what is actually on the filesystem - changes made by one operating system will not be propagated properly to the other and you will start getting files overwriting each other.

All in all a horrible mess.

Now, if you had 2 operating systems installed on 2 different partitions (dual booting) you could use qemu to boot one of them inside the other, but you must never ever ever boot the same OS twice (unless it's a read-only OS like a live CD image for instance)

You can safely run this command by using the -snapshot switch.

qemu usage:
-snapshot write to temporary files instead of disk image files