I don't know if you misunderstood me or if i'm misunderstanding you now. I'm a bit shot to pieces right now.
My "fear" was that a clinical trial with bad results could move us further away from proving XMRV is the cause of ME/CFS, even it it was the cause. It could be further ammunition for the critics/skeptics/non-believers/denialists, how ever one likes to call them. So maybe better rally prove the association first, by spreading the knowledge to more groups, like it has already happened with IrsiCaixa, Bieger, Maureen Hanson and maybe others. If association is proven, a failed clinical trial would have less bad consequences, probably.
So i don't understand the "proven" in your first sentence at the moment.
What i do believe is that if causation was proven, ARVs would be prescribed immediately.