Will, one thing you don't mention with sleeping bags is that for those of us who are cold sleepers (many, though not all, of us in that category are female), a bag rated under EN13537 testing at 30 degrees F for the "lower limit" is more like a 38*F bag (the "comfort" level). That means that those of us in this category really do need a 20* F sleeping bag.
When I first started lightening up (8 years ago now), I bought a 32*F bag (Marmot Hydrogen) since everything I read stated, as you do, that a 30* bag bag is fine, adding extra clothing if it gets below freezing. I soon noticed that I started having to add more clothing layers when the temp got under 40*F. By the time the temp got to 32*F, I was shivering even with all my extra clothing on. Interestingly, once Marmot started using the EN13537 ratings, the "comfort" level for the Hydrogen was 38*F while the "lower limit" was 32*.
The same is true of my current Western Mountaineering Ultralite, except that the extra clothes start to go on at 28*F (which, I've since found, is the EN13537 "comfort" level for that bag as listed on UK websites) while I can (with a warm enough sleeping pad) manage 17*F with everything on including a vapor barrier suit. That's the "lower limit" for that bag as listed on UK websites. It appears that the EN13537 "comfort" level is accurate for the sleeping bag I need, and that rating is about 8-9*F higher than the "lower limit" level. For me, the "lower limit" is the level at which I'm still warm, just barely, with all my warm clothing on including a vapor barrier (non-breathable rain gear). I've often seen high-altitude temps go down into the teens even in early to mid August in places like Wyoming's Wind Rivers, where in many places it's a long, long slog to get below timberline. Of course beyond mid-September, such nighttime temps are frequent, here in the Cascade Range below timberline.
Weight-wise, the extra insulation needed for a warmer sleeping bag weighs less than the additional weight of additional insulating clothing. The warmer bag just seems to me a more efficient way to provide the additional nighttime warmth I need!
I'm glad to know that I'm not the only one lamenting the demise of KookaBay. I just hope the lovely pad I have from them (3.5 inches thick, R5 insulation, 12.9 oz.) holds up a few more years! Since I can't get comfortable on a NeoAir (its horizontal baffles "buck me off" every time I roll over), if my KookaBay pad dies I'll have to go to Exped at several ounces more. KookaBay filled a need for a light, warm, cushy pad that nobody else seems to want to meet.