Prior to my 22' Intex that I left up last year, I drained all my smaller ones every year. I didn't take them down but would leave about 2-3 inches of water in the bottom for weight so it didn't get blown away. When that 2-3 inches of water thawed in the spring I had all kinds of Crud in the pool, water, leaves, dirt, dead bugs, pine needles, anything that would get blown in over winter. I used a shopvac every spring to suck up the water and gunk in the bottom.
I would put the shop vac on a milk crate and thread the bottom drain cover of the vac on loosely, just enough to prevent the stuff from coming out and to get a good vacuum on the hose. Then I could just twist the cap about 1.5 threads and it would drain out.
After sucking all the water out, we would scrub the entire surface with bleach and rinse it out with the hose. Fill with water and swim. We didn't do much more than float a puck around in the water back then. The smaller pools, we drained and refilled several times through the summer since they were small sized.
I did this with my last 4 pools until this past winter where I actually winterized the 22' Ultra Frame and left it full (right below the skimmer port) all winter.