I think everyone can be competent and good at what they do, JLC or not- for disputes or otherwise.

But there is a reason why some firms hire only former JLCs- Duxton Hill Chambers, some boutiques, etc. I mean they literally wrote the judgments for the judges- so they know what arguments/points of law are the most convincing in court. Apart from that, there is obviously their connection to the various judges (there being 40 JLCs at any one time, consisting of 2 batches) where the judges may unintentionally/inherently be prejudiced to bring in their favour.

That being said, I wouldn't say there's "no point being in disputes if you're not a JLC". Only 20 ppl from a year's batch (including overseas unis, notably oxbridge) are disputes at any time. The rest of the cohort aren't, and it doesn't necessarily mean you won't succeed.