So I found myself thinking about the purpose of the Morbuzahk. It was created by Teridax (for a change I'll call him that to avoid confusion) as part of his great plan to capture the Matoran of Metru Nui.
We were explained in one of the books I think, that the task of the Morbuzahk was to drive the Matoran towards the Coliseum during a long period of time. However, even after it is destroyed, Teridax completes this plan by summoning the Matoran to the Coliseum. He was under the guise as Dume, so that was not so hard. Plus, the Vahki could capture all of those Matoran who did not show up, in case there were some who didn't.
But Vakama points out an interesting thing in one of the comics, "Enemies of Metru Nui". He's wondering why Matoran are still disappearing all over the city despite the Morbuzahk being gone. As you may know, Matoran had started slowly disappearing after the Morbuzahk first appeared in the city. Later we would learn that these Matoran were in fact taken in Matoran Spheres before the others.
The common explanation in the city was that the Morbuzahk took the Matoran. This brings me to the suggestion here:
I think the Morbuzahk was created to serve as a distraction Dume-Teridax. It was created so that while the Vahki actually kept locking Matoran is oheres, the Morbuzahk would be blamed and Teridax would be spared of any doubts. It made his masquerade as Dume more convincing and gave the disappearences a logical reason which the Matoran believed.
My guess is that the Morbuzahk had been created with a limited lifespan as other Teirdax's experiments had been (as told in Maze of Shadows) so that it would not have got the chance to "rule" the Matoran as Teridax had promised it in the first place.
So the whole creation of Morbuzahk was just an excellent distraction which did work, seeing as it occupied the Toa Metru for a while and drove many people off course.