I don't think there's anything particular about rhubarb that will deter weeds more than any other covering, it's just that they are a big leaf that most vegetable gardeners will have a good supply of anyway. Putting them down will provide a temporary physical barrier until they compost down (which they do surprisingly quickly!) ... If you use them then it will probably inhibit germination of new annual weeds, but perennials like mares tail or bindweed will just push through our creep round the edges.
Rhubarb leaves will attract loads of slugs and snails who will shelter under the leaves in the hot weather ... So it could provide an annoying safe haven for them... Or you could check under and dispose of them on a regular basis!