Wow, just when you think that this topic has been discussed to death, it pops up again and again and again
In addition to what DevonDiver has said: You'll definitely need a pony bottle if you are diving in an area that is known to have fishing nets, fishing lines and/or lobster pots. The danger of entanglement is always there in these areas. Trying to "untangle" yourself from fishing nets, lobster lines or simple fishing lines can take a very long time whereby you can exceed all anticipated air/gas limits and it would become necessary to use the pony bottle.
One matter I take an issue with is that some people advocate relaying/planning/counting on the buddy to assist them especially when in OOA situations. I believe that the better dive should be a self-sufficient diver who can take care of himself independently including in OOA situations. An appropriate/adequate independent air supply is one critical part to achieve self sufficiency.
For the OP, there has been numerous discussions on this matter on SB. It would be worth your while to find these discussions. You'll have to be open minded when reading the discussions and sorting what is relevant and what isn't to make your own "independent and informed" decision on which way to go on this subject.