Ubiquity fails to mark "to format" a root partition when just changing the mountpoint

Binary package hint: ubiquity

Steps to reproduce (tested in the 070719 daily build):
1. Run the Advanced Partitioner in Ubiquity (Manual)
2. Select an already formatted partition and choose "Edit Partition"
3. Change the mountpoint to / (don't change anything else) and accept the changes
4. Press Next to proceed to the next step

The installer complains that the / partition hasn't been marked for formatting and doesn't allow you to continue. I guess the error is intended for (A) users that may think they can use an existing partition without formatting it, but on the other side is also annoying for the people (B) who just want to use the same partition with the same settings (size, filesystem, etc).
The current "workarounds" for B include: changing the filesystem, deleting the partition and creating it again or another or something that actually changes the partitioning.

I think the proper behaviour should be showing a warning allowing going back and continuing. From my point of view, that would be appropiate for cases A and B.

This bug is probably also reproducible for other mountpoints that need formatting (like /usr).