The great thing about using a "hybrid" setup is that you can tailor it to the way YOU want to work.
Me personally? I think that analogue volume control makes a massive difference when using a summing mixer. I always take the principal elements of my mix out with the fader at 0 in Logic and control their volume with my XRack. Why? This way I use the full bit depth of the signal. To me, a fader at -18dB in Logic sounds rubbish. Recall? No problem - it's just 16 volume pots and 16 pan pots. Takes 2 minutes using the TR in the rack.
However, many others will say that they use their summing mixer for "vibe" and integration with other outboard, and want to do all the volume control in their DAW. A "straight" summer is the best thing for them
We make modules for the XRack to work in both ways. Roll your own, but the sonic footprint is SSL either way.
There are many ways to skin a cat...
It's all about finding the cat-skinning technique you prefer.