They do level past 85, all the way up to 99. Whether it's worth leveling one is up to you. There is no new equipment, so they will drop dead when a lvl 90 monster as much as looks at them. If you have spare dollars, buying a Spirit Shaman, which will sooner or later appear in the Galleria, might be the less troublesome solution. It doesn't take exp away from you, but unlike the Kookaburra, that little friend doesn't buff you with Bless the Body.