We shouldn't confuse shared ownership and responsibility with shared access. Using the case that started this discussion as an example, there are elements of common property that aren't available to everyone – like the airspace outside someone's unit – but are still common property.
Taking this further, if you have, say, an underused bicycle storage area (shared access) which an owner wants to buy from the OC to use as a wine cellar, for instance, everyone benefits, including the the people who need to find somewhere else to park their bikes, provided a commercially valid amount is paid for the room and the money goes into the admin fund.
The 25 percent threshhold for changes to by-laws (which the sale of common property would require) is very hard to overcome if there is a well-organised objection to any such proposal. If unanimity was required, nothing would ever get done.