trackerd is the indexing backend for the Tracker package, and beagled is the indexing backend for Beagle.
So with trackerd disabled, you won't be able to search with the Tracker application.
Many users end up disabling both packages, and use their own methods for locating files, but if you use one package or the other on a regular basis (or both even), that's fine.
Gigabyte GA-P35-DS3L - Intel(R) Core(TM)2 Duo CPU E6750@ 2.66GHz
nVidia GeForce 7600 GS
2 GB RAM