Here it is:
Shooting heads are, and have been for many years, widely used for the very reason you asked about. Looking at your sig, you are a river salmonid angler. The anglers from Alaska to Washington to Oregon to California have been taking advantage of the single spool, quick change systems for decades and it's 110% true that you can go with one spool and a "pocket full of options" in a multi-head system.
The problem has always been the loops. Where river swingers don't strip the loop into the guide, striper dudes strip the damn leader into the tip sometimes. That means whatever clunks in has to clunk back out again cast after cast. So if the loop was made very slim and slick, there would be no real reason to complain about the ability to configure on the fly.
I have to respectfully disagree with the whole line change method, I have a hard enough time managing lines on spools in my fishin' room never mind on the water. I prefer to use lines over 100ft long and that is just more than I want to be winding on and off on the water.
A simple 30ft head fits in sleeve number #3 in a leader wallet right next to it's pretty sister the floater and it's ugly cousin the deep water express on the other side
My point is that it's not easy to carry several fly lines along where it only takes a good leader wallet to carry several shooting heads.
I agree that the whole line option eliminates the loop connection. But once again that damn loop is the problem not the "hot swap" on the water.
So focusing on loops... IMHO the Airflo loops are among the best in the business, and with a little practice building your own compact loops is not difficult.
If the loop slips thru the guides with only a slightly noticable click then you have it made. If it gets caught up in the guides with a fish on, you have a problem.
This is the year of the shooting head for me. Of course I will still use whole lines, but with a greater percentage of my fishing going to two-handed rods I want to be able to configure things on the beach.
One example - poppers. With a shorter two-hander, say 11ft you can throw a 12wt floating line and a huge popper 80-100ft with one backcast and no effort. That means fun with bangers when the fish are busting.
I never use a floating line in striper country because my spools are full of intermediates and sinking lines already. With the shooting head option I am going to have two different grades of sinking, a clear intermediate and a big-ass banger head (floating) in a single leader wallet in my short pocket.
The new running lines are spectacular as well, for instance the Airflo Polyshoot XT which comes 150ft long. Do you think Tim Rajeff is trying to tell us something? At a recent meeting, I complained that the running line is too long because I loved to cast into the backing and no one can do that with the XT. Then I looked to the right and saw Mark Sedotti and said "well almost none of us"
I have fished interchangeable heads in steelhead country since the 80's and will admit I was turned off by the loops clunking thru the guides and went to solid lines. But full lines give you exactly one option and require either a spools change (fast but requires much money) or a full line change (not something I want to do with fish busting in front of me).
A shooting head, given the loop thing is solved, is a fantastic solution.
I was weaned on steelhead and salmon out west where sinktips rule. The grainier belly sections of lines when combined with various tips are a great combination because they give you the best of all worlds - castability, mendability, and depth control.
The floating belly allows mending of the line into the head portion, while the sinking tip (which can be exchanged for different densities to suit the pool) controls the presentation depth. It's really an ideal system for swinging flies in current sub-surface, and of course with the loop system you can always put a floating tip back on and fish bombers or skaters.
I never fish indicators or split shots on the leaders so am unable to comment on the effectiveness of lines for those techniques.
Summary:
Sink tips that combine floating heads with various density tips are perhaps the optimum system for swinging flies for salmon and steelhead (most G/L anglers are not swinging but drifting flies). This is the dominating approach on the west coast, thus sinktips and floaters are 99.9% of the lines being fished for steelhead west of the Mississippi.
Shooting heads are very effective for configuring on the fly, but unless the loop system is made to be of minimum resistance in the guides that poses a problem for full line strip retrieve fisheries like the stripah. Sinking shooting heads snag up too easily when swinging flies in a rocky salmon river and are rarely used in the pacific northwest as a result. Tips (above) are the preferred method there. Back here on the striper coast, IMHO the only problem is with the loop, but better loops are coming soon.
Whole lines are cool, I use 'em too... but I am on a mission to give shooting heads a total workout this year and find/make loop connection systems that are hardly noticable going through the guides. Frankly, if a loop connection is undetectable going through the guides then I really can't see any reason why the majority of anglers wouldn't use them for coastal flyfishing, but that's just my .02