Private offices aren't cubes. You can still hear conversations with cubicles. And if you have an office with more than one person, the long conversation still disrupts the other people in the office.
Long conversations do deserve private space, but this can be resolved by having ample meeting spaces.
You can hear loud conversations. Those fabric walls do a surprisingly good job at limiting the spread of a normal-volume conversation to about 1 cubicle around you, and you can speak in soft tones and not be heard at all.
Long conversations do deserve private space, but this can be resolved by having ample meeting spaces.