Actually John, i usually have both rods protruding quite far out, years ago whilst fishing right in the nearside edge, i had a take which as i bent into the fish was immediatly cut off, my line must had gone under something - a root maybe who knows, but since then i've put my tips out as far as i can get them, even when fishing right in the edge so my line is angling toward the near side from further out, reducing the risk of that happening again, it was pretty much a freak i know, but is a confidence thing with me.
The back lead i think is a matter of personal preference jon, i usually like to back lead much further back than a flying back lead would give me on the river bed, mostly for reasons of the Barbel giving me liners which i'm always worried will spook a big one, but also from the point we've just been talking about.
Thats just my preference, using flying back leads is easier though if you don't mind only a few feet of line pinned down, i always worry that if a big one hits my line, it doesn't matter whether it's 6 feet or 16 feet from my hookbait, it'll be on it's toes pretty quick, but i know Paul Monaghan uses them, and you can't get better results than him
so maybe i let my paranoia get the better of me !!
Ian.
Edit ...Just reading your post again ... how do you mean a semi fixed back lead ? i assumed you were talking of a flying back lead sorry !
The only time i've ever used such is by threading a drennan float stop up the line and wrapping heavy metal putty round it, which has it's uses when on the rare occasion i'm fishing very very close to my rod tips, or even rarer when i feel the nature of the river bed is too risky to backlead too far back useing a conventional backlead.
They can be a bit awkward to cast especially any distance with accuracy, and the further you have them up your line away from your hook bait the more problamatic they become, not really an issue if you don't need to be that accurate with your cast, but then i wouldn't be using them for casting any great distance.
If when casting a reasonable distance, when i thought it would be unwise to back lead, any great length, i would then use a flying, or not bother at all, when casting far enough, if the flow is gentle enough, a sinking line will usually give you just as much line on the botom anyway.
Though i prefer to use a clip on back lead, i like to keep my options open, but i'll usually choose swims where i dictate the method