REO fits in hand and includes a 3ml bottle and 14500 battery in the Mini, or 6ml bottle and 18650 battery in the Grand. Most tanks are nearly as tall or taller than the height of a REO itself and most tanks 3.5ml -10ml will turn the juice to tar before all the juice can be vaped. Bottom feeders, that work, like the REO's you lightly press the bottle every 8-12 toots to wet the atty again and the juice stays clear because you are only using the amount you need when you need it. But REO's have the same problem that make REO users want a rebuildable atty as those of us who already use rebuildable atty's we want an atty that will save us money and usually last longer than a couple days. Genesis style attys are great but they defeat the design and purpose of the REO and would also make the REO as unwieldy as using on on top of a tube mod.
I don't know about most vapers but when I'm out in the middle of nowhere for days at a time I want a mod that is built to take punishment and that I can fix with what's in my pockets. When you're out in the middle of nowhere every ounce extra you have to carry means more potential for accidents and could mean the difference between surviving a situation or dying in one. When I'm out in the yard I will not and cannot carry a pack that weighs more than 20 pounds and that includes my boat hammock, food, cook gear, sleeping bag, spare layers of clothes and a 3L platypus bottle because water is scarce where I live. More weight to carry equals less mobility, less mobility means less stamina over time. The less you carry the batter prepared you are to hike/run/climb longer and farther. An ounce of tobacco used to give me around a carton of smokes add a 20gram Bic lighter and I can handle that weight because I was a smoker. Now with vaping carrying around a large mod and spare batteries and a 30ml bottle of juice along with spare attys cartos is first of all more complicated, second more to keep track of, third more weight than tobacco and a Bic. The less my mods and all associated gear weighs and the less space it has to take up in my pocket or pack the more likely I am to travel longer, happier and safer.
I love my GG but there is now way that I will take a 1 pound chunk of metal out into the woods with me. I love my Precises but they too are way too heavy to lug around for extended periods of time. A REO Mini uses batteries that I can charge in my .5oz solar charger along with the other batteries I may need in the wild. 3ml's is roughly a day's worth of vape and it fits inside the REO, add a large bottle of juice depending on how long I'll be in the woods and I'm almost set, except for undependable attys/cartos which I'll need to take at least twice as many as I think I should. Now with a rebuildable bottom feed atty all I need is a little mini ziplock with enough wick and wire for a few rebuilds which equals about the same size as a single atty or carto, less stuff, less weight, more mobility. Are you beginning to understand the reasoning?
If you sit at a computer or don't get out and about much then mods like GG's are great mods, but if your life depends on the weight you have to carry a GG with a ScubaGen on top is about the dumbest mod to take out into the wilds.
One last time, "The less you have to carry the more you can do." And, "If you can multipurpose what you carry you can carry less." I have a pocket knife I carry at all times. It is small only has 3 blades but with it I make my living as a carver, I can perform minor surgeries in a pinch with it, and I can skin and gut the animals and fish I need to survive on while I am away from civilization. Most other campers that I have known think they have to carry three or four knives usually with blades longer than they'll ever really need to use and they can't carve squat with them when sitting around the fire at night and I sure wouldn't let them near me with a 10" blade if I needed an emergency surgery in the middle of nowhere. Now do you have an idea why some would want a rebuildable atty for their REO's?