I would just erase it and reformat it for MorphOS. I still have dual boot option on my Mac mini and it never gets used.
While on the mini I share your experience, I think on the Powerbook a dual boot setup is rather handy. When at home you usually have choice of several computers and no particular reason to fire up OS X on that certain machine which is running MorphOS, but when on the road you usually have only one machine with you. Then it's nice to have a choice. As much as I like MorphOS it also has some software shortcomings which can be bridged by OS X.
But I suggest another dual boot setup (that's how I did it with my mini): Get a firewire hdd. Install MorphOS on the internal disk and OS X on the firewire disk. The fw cable is you boot menu then. If plugged in OS X starts, if not MorphOS boots straightly. A very clean solution. But requires the external drive which is no issue for a desktop machine, but kind of an issue on a mobile device.
I am not yet decided myself whether I'll use that solution for my powerbook or if I am going to do a dual boot on the internal drive.