I'll be curious what experienced pool owners say with regards to your question. In our new pool, the valves are set to suck equally between the main drain and the skimmer. If it sucked only from the single skimmer I've got, it would suck both water and air, as the suctioning is strong enough to do that (even running my pump at 80% of max). I know that because the first time I backwashed my DE filter, I tried sucking from the skimmer alone (where I added the new DE), and was a little aghast to see the skimmer completely empty, and realizing that the flow into the skimmer basket wasn't fast enough to replenish the water being sucked out of the skimmer. So, it's half-n-half in my pool.

One thing I like about this is I hardly need to vacuum my pool. I've done so three times only since I got the pool (8 weeks ago). Whatever debris collects on the pool floor appears to, eventually, get sucked into the main drain, as my water is crystal clear.