To address 2 points:

If a stereo bass sub config is used with a MC format, mixing the mono .1 signal equally into both subs will collapse the stereo effect.

As was pointed out, this is in rare cases of source material, but still part of the equation, and likely moreso as MC audio evolves.

Summing the .1 channel with redirected bass can result in 'no problems' unless, like my setup, you have 1 sub that's designed for LFE and another sub that's designed for redirected bass.

If a sub is capable of very high output at very low frequencies (which it has to be to be capable of playing RB+LFE+10 anywhere near ref levels), it generally isn't as good a sub for redirected bass.

Conversely, if a sub is designed for redirected bass, it can't play the summed signal well, if at all.

If you seperate the 2 signals, like I have, each sub has less output requirement, allowing for money saved in each sub's case. Also, as I've said, generally, the redirected bass sub has a higher F3 and is designed to roll off at 12 dB/octave while the LFE sub has a lower F3 with a steeper roll off to protect it from damage.

So, it isn't a matter of having no problem summing RB and LFE into 1 or more subs (obviously it 'works', or none of us would have our sats set to 'small') as much as it affords less flexibility in sub selection and reintegration of redirected bass.