Friction shifters will almost* always work. If your rear der brakes, and you have to replace it with something you find in the middle of nowhere, odds are you can ride with friction shifters and that for a while. If indexing gets out of tune, you may have to take it in to get looked at, not so with friction.

Looking at those bikes though, the bar ends are indexed for the rear and friction for the front. Normally they have an option to switch from friction to index, but index for the rear is common.