Our pool has main drains. For the first several years, my cleaning process was to brush the pool, moving all the dust-like debris towards the main drains. Then I'd put in the pressure side vacuum. Even after vacuuming, I'd still have that dust-like debris on the bottom no matter what.
Now I have a robot and rarely brush. But also have almost no dust-like debris.
A few observations. My filter cartridges the first few years with brushing and a pressure side cleaner were quite dirty. Pressure indicated I definitely could not go all season with less than a 25% rise in pressure. Now using a robot and not brushing, but having a dust free bottom, pressure rise indicates I could pretty much go a whole season in MD without cleaning the filter. And when I did opt to clean them last weekend, they were noticeable cleaner than before.
So although you will get a bit more circulation with a main drain, my experience is that with a low efficiency (bad cleaning) vacuum (pressure side cleaner), there was more benefit to having a main drain (sucked the dust out that I stirred up). But with a robot to suck out all of the dust, I no longer have the need to have a main drain to push that type of debris towards. That said, I still appreciate having the main drain for circulation, though my pool is 8 1/2' deep. Though I have no real feel for how much difference it makes. But the drain sucks in as much water as a skimmer, because it has the same size pipe and suction pressure would be the same.
Not maybe what you were asking for, but some details on my system and thoughts.
