The main drain is a high failure part of the pool, and it's usually expensive to fix if it has problems. There are some studies that indicate it doesn't contribute as much to circulation as people believe.
Because of the 'high failure rate' of MD's the company I work for installs 2 'low suctions' in our pools. A low suction is simply a suction line ~ 28" below the water which will 'pull in' the water, it's not as effective as a MD, but you can fully winterize it! (I think Dave has 1, because he asked about a special cover for it last year).
There is a benefit to having a suction line lower than the skimmer mouth - if you want to lower the water below the skimmer, you have a line that will do that (yes, I know the ways to do it if you only have the skimmer :blah: ).
If you've any questions about the plumbing of you impending pool, ask the nice folks here and I'll bet that you will end up with a better circulatory system than your PB has planned.
i have one of those suction lines, with no main drain, its about 3 inches lower than the nearest return, but i have a diving well, you think they would have put in a main drain. is there a cover for it. but to answer your question, i have no main drain and i have a 9 foot deep end, so there must be some logic to it.
I have a vinyl with a main drain.. so far so good, knock on wood. Where I find it helps is if I am just brushing I can direct some of the dirt to the main drain without having to get out the kreepy or vacuum. It also helps to determine the clarity of the pool, its white so when it looks hazy I know something is off. On opening the pool, I can run the pump as I am filling without having to wait till the water reaches the skimmer. As for mixing chemicals, I am not sure, logic would say that when I am pouring in acid or chlorine and it sinks to the bottom, it helps circulate it faster through the system. Oh yea, when I am diving, its something to aim for