Since some rigs can require a lot of scrolling to move around in the Timeline, having multiple Timelines for this sort of thing is a great idea.
Certainly much better than putting the rigs into folders, because:
Expanding/Collapsing the folders' contents constantly is tedious, especially with particularly large/complex rigs.
When they're expanded, you now have even more scrolling to do due to the additional folders on the Timeline.
So instead, switching between different rigs to animate is as simple as one click, to switch between Timelines.