Leaned to dive in 78 - only instructors had octos. There if you needed it during training, sort of a backup in case something went wrong with buddy breathing while learning the skill.
Did not own one with my first set of gear, didn't miss it, but that was then.
I can't imagine diving without one now. As others have said your buddy has been trained to expect it. To try to buddy breathe without training and practice is a disaster just waiting to happen. Cheap insurance, wouldn't be without one - keeping in mind that you are wearing it primarily for your buddy - not yourself. Except that your buddy might just take your primary from you if OOA then you will be very glad that you have the second reg.