It is a slow day in Mr V's land, so I'll bite.
Dealing with entanglement is a skill. Instead of saying "I need extra air in case there is an entanglement" say "I need to figure out how to avoid being entangled."
First, you should avoid any entanglement by not diving beyond your skill level, e.g., do not dive wrecks/caves unless you're certified. However, it is still possible to come across a fishing line or something else. In that case, it is up to you. You can choose not to go where there entanglement is possible.
Second, if you must cross a line, do it over the line and not under it. If you get entangled, you should make one attempt to free yourself and then signal your buddy. A diver is more likely to make entanglement worse if the diver continues fight the entanglement. Cave and wreck divers practice this skill and it is better to experience the scenario in practice than "understand it" via a forum post.
Third, let's look at an entanglement scenario at 100ft. How long do you think your 40cf bottle will last you when you are fighting for you life? I'll let you work out the math but will say that those few minutes will be the longest minutes of your life.
So back to the original question in the thread: are pony bottle dangerous? No, they're not but they may lead some divers astray. If your dive plans raise concerns about redundancy and dealing with emergencies, get sidemount/double training, learn how to dive in wrecks. And no matter what you do, dive conservatively. Finally, look at the incident statistics. Equipment failure is not on top of the list.