A Concrete floor is important, if you want to be efficient and have more than 1 guy load. Inexperienced operators tend to dig down rather than pull in and curl up. Our mulch bins are on a concrete pad, and we keep a couple of floater bins there - one for topsoil in the summer, sand/salt in the winter, and 1 has crushed granite in it now, but might have belgium block, or other at any time. We keep stone on the ground without a concrete pad - basically I got tired of dropping $1000 a load for concrete. You have to be a little careful for clean drainage stone, but after a bunch of loads you sort of end up working off of a stone base. Same thing for the processed and dust. For topsoil, if we don't load the bin, and just keep it piled at the screener belt, we end up scraping up the base of the pile every now and again to run it through the trommel again. There are just so many times you can drive on soil before it packs hard.
Also - If you have the room, make the pad extend beyond the bins so any extra material that falls of of trucks can be tucked back into the proper bin.
I don't think that is big enough. Our smallest bin is roughly (inside dimensions) 12'x30'x8'. I mentioned this before in a related post, but if you are interested, go to my Soundview website, click Company Info on the top left, then Soundview pictures on the right side of the next screen. You'll see pictures of the nice bins and the cheap bins in there.
A couple more comments -
1. You originally asked about other benefits than loading out of the middle. Concrete will help keep the product saleable after rains rather than have it floating all around the yard.
2. If you put the block walls at the back edge of a concrete pour, figure that it will eventually break the concrete off. We used 6" of 4000# concrete, and the back wall of the nice bins held for about 5 years before cracking off. Now we temporarily put the back blocks inside of each bin until we repour a back section with a footing or some other engineered idea I come up with while on the thinking chair.