What, if any is the "down side" to a main drain???

Sorry, I already stated the rest above everywhere... You have yet to address that single point.

As far as it being a possible "leak point", again, no more than any other hole put into the liner for any other purpose (such as the skimmer/returns/etc). As far as it being an entrapment, again, dual drains far apart = no entrapment hazard, especially when cut in with the skimmer.

Mark, I fully respect the engineering side you champion, and I am usually the first and foremost to do function over form. In this case it is just common sense based upon the usage I have and the fact that it is drawing water in from the center of the pool at the bottom. That in of itself overrides any other "noise" from arguing against it. It allows for complete and total turnover of the water in the pool constantly with the best mix possible within the shortest timeframe. Which is primary for heating a pool, cleaning a pool and even sanitizing a pool as far as I am concerned. Oh, and I may add, that yes the main drains to cause enough suction to pull the solar cover down in the center if there in rainwater sitting on top of it. In those cases we go out and roll the solar cover up and refloat it after to get the extra rainwater to mix with the pool water.

Outside of entrapment with older single drains, I fail to see any sort of downside of using a main drain. It offers far more options and flexibility with absolutely no downside whatsoever if used with proper valving. I still fail to see the argument against them (outside of a pool builder/maintainer not wanting to deal with customers with them). If I don't want to use the main drain and want max surface cleaning effect, I can just switch it off (and I have valving at the pool and at the equipment pad, so I can do it where-ever it is convenient). Conversely I can also switch off the skimmer if I want the main drains to do all the work (and I sometimes do that to optimize the pool heating).

And, if you consider the installation of a main drain as the big reason not to have one (which for the majority of people the reason they put up above ground pools is because they are easier to do and a main drain negates some of that "ease"), in my case that is not the case at all. I have autistic children that I would worry about constantly if I had an in-ground pool. For me it wasn't even an option, so I put all the piping for my pool in-ground, hard piped everything with 2" PVC and located the equipment pad at the back of the garage and built a 1200sq/ft paver patio around the pool, so putting in a main drain was trivial in comparison. This way I have zero concerns about the kids and the pool piping and anything else concerned in the backyard. It is fenced in entire with wrought-iron fencing (real wrought iron that is welded and solid) so I can let the kids out in the backyard with no concerns at all. I did all the work, including the patio, so an in-ground would have been a cakewalk to do at the time, but the worry with the children didn't make sense, so an above ground was the only option.

Besides, I can always pull the pool and fill in the area with dirt, a fire pit, gardens and sitting walls when we no longer want to have a pool. I don't have to touch the paver patio or the rest of the landscaping when we get to that point and it all still "fits"...