I haven't been diving long, so my dives are fairly short (SAC only really allows me 30min or so), but I'm yet to get cold in my 1piece 7mm in ~50F.
You've clearly also not been diving long enough to recognize that "I've yet to die of hypothermia" is not the same as "I've yet to get cold."
:cool2:
But seriously, there is a very real physiologic difference between "I didn't get cold" and "the cold didn't bother me." Cold water (50F water is cold) exerts specific negative impacts on your body...whether you "feel cold" from a comfort standpoint or not. If you and another person of equal size and wearing similar thermal protection are put into 50deg water for 1/2hr, even though you might not "feel cold" your core body temp will drop at the same rate and to the same temp as the other person, who may be shivering violently. Whether it bothers you less than the other guy does not exempt you from the same laws of thermodynamics; conduction cooling is a function of water temp, body surface area, and time.
Because of that, the fact that cold water temperature "doesn't bother you" as much as other people is actually a bad thing. While others' bodies are telling them "it's time to end the dive" long before they get anwhere near having a problem, you're body is saying "c'mon in, the water's fine!" So even though you may not "feel the cold" even a mild drop in core body temp diminishes physical and mental abilities, thus increasing the risk of you having an accident AND/OR decreasing your ability to effectively respond to an accident.
Additionally, while the effects of cold water on your body are related to the time of exposure ,that relationship is non-linear, meaning that you might be "fine" at 20min, "fine" at 30min, even "fine" at 39min, and suddenly "shivering violently" at 40min. Worse still, this time aspect typically varies from one dive to the next based on other factors - mood, hydration, stress, activity level, task loading; even things like viz, surge, currents, etc. Which means that the fact that you were "fine for 40min last week" has no bearing on this week's dive, where even though the water temp and your wetsuit are the same you suddenly find yourself shivering violently at
20min.
Lastly, you need to consider the correlative relationship between "time of exposure" and "bottom time." It's intuitive - if you think about it - that your progression from "OK to cold" starts at the beginning of the dive and by definition always reaches its maximum at the exact moment you begin your ascent from your max depth. Unfortunately this also just happens to be when you're at the deepest point of your dive, most tired, most low on gas, furthest from shore, furthest from anchor line, most likely to be narc'ed, and pretty much any other "-est" you can think of including highest nitrogen loading, max p02, etc. Even worse would be if you were actually ending the dive due to an adverse situation such as gear failure, bad conditions, buddy separation, low gas, etc. So no matter what, on every single dive, the very point in time when you need to be THE MOST physically and mentally capable of ending your dive safely will ALWAYS coincide with the point in time at which exposure to cold water will have it's GREATEST negative impact on your physical and mental abilities.
There's a reason that we use the term "exposure protection" and not "comfort" to denote the function of wetsuits/drysuits. As PapaBear said, it's a safety thing. The fact that you're also more comfortable is merely a pleasant "side effect" that you experience when your body is protected from the thermodynamic and physiologic effects of exposure to cold water.