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I would propose that we learn quite a bit from simple treatment/control comparisons. We learn whether or not the treatment has any impact at all, which is often not the case. If we provide cash or another service to the "control" group, then I'd say it's no longer a control group, but rather a comparison of two different treatments. 

But let's say we had no true control group and just compared Treatment A (say, training) to Treatment B (say, cash). If we saw no difference, then we'd have no idea whether it was because Treatment A and Treatment B were both equally effective or because they were both equally ineffective. 

So while I think multi-arm experiments do teach us the most, I'm always glad to have an actual control group so we know if the effects of the program are even positive.