Therefore, it would seem to me that a stop at 30 ft followed by another at 20 or 15 would make more sense, or even better would be to slow your descent so gradually at the 30 ft mark that it takes 4-5 minutes to surface from 30 ft. That would allow for the most gradual return to normal as possible, and it is safe because as long as you are less than 30 feet you aren't taking on any more nitrogen.
First off, the real kinetics of nitrogen in the human body are unknown. Although I believe the Navy has done some actual tissue measurements of nitrogen tensions, those data are not available to the public, and what I have been told is that they are very limited. What we use to build our ascent strategies is the output from mathematical MODELLING of nitrogen kinetics, based on a combination of the physics of diffusion and equilibration of gases in liquids, and experimental observations like Haldane's studies of goats.
What matters, it appears, is the RATIO of the gas that's dissolved in tissues to the pressure of the gas you're breathing. Haldane, when he compressed and decompressed his goats, decided that you could tolerate a ratio of 2:1; that is, if you were at 4ATA and came up quickly to less than 2ATA, you were likely to get bent. In fact, since it's the nitrogen that matters, it wasn't really 2:1, but 1.58:1, but the idea is the same. Although this concept has been refined with time, the models still use an acceptable ratio idea to control ascents.
The consequence of the ratio idea is that ascent rates have to slow as you get closer to the surface. If you look at the output from decompression software, you'll see that most of them create an ascent that has a curve to it. The shallower you get, the longer the stops, so your ascent gently curves you to the surface. What the shape of that curve depends on the assumptions that underlie the model. Pure diffused gas models send you quickly shallow and have you stay there a long time. Bubble models tend to slow the ascent deeper and make the curve more gradual, because staying deeper is thought to control bubble expansion.
The steady ascent to a shallow safety stop is a diffused gas approach to decompression. The steady stops from half max depth to the surface is more of a hybrid approach, doing some time deeper to control bubbles.
Obviously, this is a very simplistic explanation.
My final comment is that, since tissues in the body vary in the amount of blood flow they get, and in their composition, it is thought that various areas of the body absorb nitrogen at different rates. This is modeled by tissue "compartments" in decompression theory. Very slow compartments are thought to equilibrate over very long periods of time. It is quite possible to do a short, deep dive, come up to 30 feet, and still be ABSORBING nitrogen in the slowest compartments of the model. But we will never spend enough time in the water to load those compartments to where a direct ascent to the surface will exceed the acceptable ratio for remaining asymptomatic, so in essence that absorption doesn't matter to us.
