OsX has lots of hidden files, so it needs a lot of disk space (e.g. for dvd authering - ok, not on an ibook - you need up to 12 Gb (!) of hidden space!)
so, if partitioning the hd, don't make it too small!
only advantage i know is, on a sperate pertition you can while booting select your system by pressing X for the MaxOsX part or - uuh, uhh? was it...- alt?, to get a selection for what system ypur prefer to boot
so, on an iBook i do recommend NOT to partition the hd.-