The tooltip for the "Track to" constraint says: "Legacy tracking constraint prone to twisting artifacts".
Why is it "legacy"? Has it been replaced with something else?
After some further research, I found that Joshua Leung added this text to source/blender/makesrna/intern/rna_constraint.c when he committed the new Damped track constraint, indicating that Track to has been superseded by Locked track and Damped track.
Of course I don't think anyone's about to deprecate Track to.