when trying to install 13.04 in an existing ubuntu partition. the installer gives an option "erase and reinstall on existing ubuntu partition". doing that also erase all the other partions on that hardrive in including ntfs/fat ones and makes one big partition.
is that a nasty bug or a feature ? this might be reproducible easily in VM.
That is a feature - and not a bug.
Depends really on what kind of partitions you've made earlier, and what they are used for. If Ubuntu is the only operating-system used on a computer, it will remove all partitions because it wont affect other OS's.