I am of the school of NO SHARED NEUTRALS UNDER ANY CIRCUMSTANCES!!
I suggest you try some other schools.
What happens when you get to the panel?
You wanted to 'stir the pot' well you hit a nerve.
There is no reason whatsoever that multiwire branch circuits will in them selves cause a 'hum'.
Even if you do not want to believe it there was something else going on.
Electrically sharing a neutral on a branch circuit is not any different than what happens at the panel where all the branch circuit neutrals share a single conductor back to the source.
The cost of the extra wire is NEVER that great, compared to the potential problems.
Not true at all, it really depends on many factors. In a house I might agree that the savings is not that great, in a commercial pipe job the savings is tremendous.
1)Less total conductors result in smaller and / or fewer raceways.
2)Less 'current carrying conductors' results in less derating of the conductors.
3)Voltage drop is reduced on long circuits with the use of multiwire branch circuits.
It is really up to the customer or their engineer if the savings are worth the 'risks'.
Personnel IMO with competent electricians the risks our minimal.
We install predominately Multiwire branch circuits and we are not going back to fix things.