Oops. That’s a very strange idea in my opinion! Not laughably silly, but strange.
Two considerations from my technical idiots point of view:
1. Using analog summing may or may not affect the sound, especially the stereo width and/or perceived depth, for the better. But let’s keep this subjective question aside. The H9000 is a digital machine, so it can only do an emulation of analog circuits. Which tons and tons of plugins will do as well. So, if you just go from ADAT/MADI or whatever digital input to another digital output, it will actually sum channels by doing the most simple math (sample value + sample value for n times, where n is the number of tracks), not by moving electrons on the audio path. Which is considered to do the trick alone.
2. If you use any A/D or D/A conversion from the H9000, things are different. People say that they like out-of-the-box summing better. And yes, it does change the transients a lot, actually! But you don’t need a summing box for that, just a DA/AD loop to achieve the same effect. Generate a pure square wave in you DAW, send it to the loop and voilà, it looks completely different now! Admittedly, this is not summing anything, but who knows what happens on your output? I mean, you ears cannot process digital data, so just another D/A has to be done. And then you have it all. Not?
Just my 2 cents.