There shouldn't be too much debate about this. We don't "need" two ears, but the fact that we have two instead of one adds the ability to localize the sound source. There is also an improvement on clarity and fine distintions between sounds.

Given that Bose accepts that their mono system isn't able to reproduce some stereo samples with the same quality than stereo systems do, the test I would like to perform is to compare the quality of a (true) mono sample on their system with a stereo sample through a stereo system. No other comparison seems fair to me, but it will never be possible in some keyboards (namely on my Tyros Stereo Grand Pianos). That would help me to estimate if the investment is worthwhile or not.

-- José.