If someone could show a 55% hit rate over say 10,000 very well controlled tests, that would strongly suggest something interesting is going on.
Such a tiny effect would not be normally noticable, and could only be detected by a great deal of painstaking experiment. It certainly could not account for the widespread belief in the paranormal that we see; that could still only be explained by the faulty perceptions and cognitive biases to which sceptics attribute it. So why postulate the existence of this tiny effect at all? Just because we haven't yet done quite enough experiments to completely rule it out?
We have a pretty good idea of how vulcanism works, but we haven't completely ruled out the possibility that a tiny percentage of it could be due to angry volcano gods. We have a pretty good idea of what causes mental illnesses like schizophrenia, but we haven't completely ruled out the possibility that a tiny percentage of schizophrenics might be possessed by demons. Do you think it's worth devoting the time and resources to investigate those possibilities?
Now the challenge is not a piece of scientific research as such. JREF do not need to allow such small margins and cannot really afford to do so but they need to set the bar to a level that whilst weeding out chance allows real paranormal powers to show up.
AIUI applicants for the MDC are typically required to beat chance odds of 1:1000 in the preliminary test, i.e. if you test 1000 applicants you'd expect one to pass even if paranormal abilities don't exist. That seems a reasonable place to set the bar to me. But it's moot anyway; if you look at the challenge forum and find the threads for the few applicants that actually got as far as taking a test you'll find that they never do even a little better than chance.
This psychic produced readings for ten people who were then given all ten readings and asked to try to identify which was theirs. You'd expect 1 of the 10 to do so by chance, the success criteria was set at 5. Was that too high? Would 3, say, have been enough to "strongly suggest something interesting is going on"? It's an interesting point to argue but irrelevant; her actually hit rate was 0.
This telepath picked 20 words out of a list of 30 to transmit to a receiver. The success criteria was set at 19 to be correctly received. My maths is too rusty to work out the odds in this case, but that does seem a bit high to me. But again it's irrelevant; his actual hit rate was 0.
If applicants were regularly doing better than chance but not reaching the required success criteria it would be worth having the argument about whether those success criteria were being set too high. But they aren't, so it isn't.