Why alternators are used to generate DC?
Brushes and commutators which need to actually switch current will often be far more wear-prone than slip-ring assemblies which don't need to perform any switching functions.  This excess wear comes about both because it's hard to make a smooth surface with multiple separated contacts (without the space between the contacts being higher or lower than the surrounding contacts themselves), and because switching contacts under load puts extra stress on them; while a motor or generator with many windings will put smaller switching stresses on the commutator than one with fewer, a non-switching slip ring will produce none.