What you propose Clive is fairly indisputable in a certain situation, but that situation would be : casting into
still water with
zero drag exerted on the line above the lead/feeder (i.e. both lines : that above, and that below, the ledger/feeder, would sink to the bottom ~vertically). But if there is any flow then that force will have a greater sideways effect on the bait than it will on the lead. But you have variables here. That is, the effect of flow (sideways force) on the bait compared/relative to its effect on the lead would be dependant on the size/buoyancy of the bait compared to the lead. E.g... the effect of the flow on 1/4 tin of spam cast out below a 1/4oz lead would be much greater than the effect of the flow on a single Elips below a 2oz lead. And that's not taking into account any drag on the line above the lead/feeder, drag that will take that line off its trajectory towards the river bed. That is, a lead/feeder cast out on un-feathered line will fall to the bottom almost vertically ("almost" as any current will result in it having a resting place on the riverbed at a distance slightly downstream of where it hit the surface, that distance being dependant on the flow, the weight cast, the weight's resistance to the current [feeders obviously exhibiting more resistance], and the depth of water). But if the line is even slightly feathered then the course of plummeting line above the lead/feeder will not be parallel to the line below the lead/feeder. In fact, the line above the lead/feeder will be at an angle away from vertical towards the rod tip, and the lead/feeder will fall in somewhat of a arc if the cast is feathered hard (i.e. on a virtually tight line). Then you've got the slight drag on the line above the lead/feeder by the river water itself.
But all that, and much much more, is by the by, and of little or no import. What is important is, if drag is applied to a cast line (via feathering) then the line below the lead/feeder should be in a straight line, so that line should not tangle
with itself. Yes, if the water is shallow, if the hooklength is long, if the current is slow, if the bait is light/buoyant, then the hooklink could possibly come into brief contact with the line above the lead/feeder. But you'd have to be unlucky for them to actually tangle together.
Basically, given a certain situation then I'd agree with you Clive ... but what Rich says is easier to agree with. That is, feather down and you'll get a lot less (or no) tangles.... and the rest is theoretical bo**ocks