What’s the rationale of running a drop leg holster or sub-loads… other than it looks tactical/tacticool? Is there a practical application for civilians specifically to carry gear in this manner? I mean really who carries their edc in this manner, and why would someone train one way with their gear placed in a sub-load and carry their gear differently in the real world? Someone please explain.
My take is that 1) primarily drop leg holsters lower the sidearm below the bottom of heavier, externally worn ballistic vests; 2) some officers use them to get the weapon away from the equipment on their increasingly crowded waist - many agencies still don’t allow officers to wear external vest carriers, some that do allow the practice don’t allow officers to mount equipment on the vest; 3) a small number of officers wear them because their duties involve operations in areas where a hip holster might snag - searching vehicles might be an example, a lot of K-9 officers wear them and I’d guess a lot of police aviators wear then to avoid the hassle of harnesses - but that is a WAG; 4) everybody else - ‘hey, look at me I’m an operator.’
Other than the first couple applications mentioned, the problems with drop leg holsters out weigh any advantages - they are slower to draw from; drawing from them in some positions is more difficult than the normal holster - but you can probably think of examples where the drop leg is easier to draw from; weapon retention is more difficult; and running with shit flapping around on you leg isn’t all it is cracked up to be - IMHO you cure that by wearing them as high as you can and still draw without interfering with any equipment.
I really can’t think of any real reason a civilian would ‘need’ to wear one, maybe someone else can chime in.
As far as training - some people just don’t think things through. 
The only sub load I still utilize is for my gas mask. All other items are kept higher to my waist to eliminate movement while I am moving while also being more ergonomical. If you must get the holster lower, you can utilize a lower drop UBL in the case of a Safariland holster or even a drop leg, but one that rides higher and uses only one strap higher on the thigh. Before we learned more about the use of the drop holsters, we could battle ours swinging to and fro for hours while on hikes or field ops. Eventually we either jacked them up higher on the thigh or simply with a lower profile belt mounted holster.
True for the most part.
I’d add that pretty much any situation where a belt holster would pose a snag hazard for an LEO working around vehicles, would be worse with a drop holster.
WRT K9 officers, I’ve only seen an officer get snagged on a lead around the waist a couple times, yet I’ve seen dogs’ leads get wrapped around thighs, ankles, and knees more often. This would suggest that the thigh holster would be more of a problem for K9 officers than belt mounted ones. I’ve heard this argument before, and never understood it.
I surmise that this method of carry became popular with the K9 folks back when belt mounted holsters that accommodated WMLs were more scarce, and so K9 cops who wanted WMLs had fewer options to pick from.
The only practical use on duty I have is for extra ammo while practicing. I can squeeze one mag into my external vest carrier as it wasn’t really designed for AR mags. I can’t do a battle belt as I already have a duty belt.
For concealment purposes. Something like the costa leg rig is easy on and off. For the most part it doesn’t it doesn’t interfere with my concealment mag holder & gear when I practice. I load it up with a bunch of pistol mags. I still load from my concealment mag holders. The leg rig is there to lessen the trips to load/grab mags.
It also packs easily in a rifle case. If in some rare chance you had to deploy a rifle, it’s a quick way of adding a few rifle mags. The leg rig won’t interfere too much with rain gear and won’t be too visible if you’re driving. If your a prepper, it’s an easy way to suit someone else up as one size fits all.
Having your sidearm clear your body armor was a big reason for the drop leg holster. I run a ALS low ride at work and it is perfect for me, I would not want my handgun any lower than that. As far as sub loads, our new chief put us back in blues and said no more outer molle vests where my most of my gear went to include, radio, taser, rifle mags and blow out kit. Since my belt has all that crap on it now I run a buckle on the belt and hook up a safariland drop leg platform with rifle mags when I deploy. It sucks but beats throwin mags in a pocket.