I don't have any qualms with 100 feet being a good rule of thumb to stick to and I agree with you on setting personal limits from an educated stand point. It's part of knowing the risks, understanding your personal physiology, and proper preplanning on a dive. That wasn't the point that I was trying to make, but I agree that there's a slippery slope there and that saying "well, what's one more foot" over and over can quickly get you in trouble.
The point I was trying to make is that setting those limits is based on personal risk tolerance, understanding your personal physiology and proper preplanning of dive. The right number for you is 30m. For someone else, that number may be 20m, and for another it may be 40m. It's based on experience, conditions, who you are, etc. Are those guidelines established for a reason? Absolutely. It's much better to understand those reasons and act accordingly than to blindly follow a number that's been set and then justify it by non-nonsensical statements that only mean something at first glance.