I agree with you that internal regulation can work. However, a internally regulated alternator cannot usually properly account for voltage drop to the battery. A typical approach is to set the voltage a couple tenths of a V higher and hope that cable losses account for the rest. A slow slide is a nice general approach, but it does not offer the profile control that a modern 3-step alternator has.
If I recall correctly (and I bow to your industry experience), some of the Dynasty battery literature I read mentioned that different charging regimens led to different product lives (constant voltage vs. other ones). Naturally, trickle-charging was the least damaging while rapid recharges attacked the positive plates. I doubt Dynasty, a battery manufacturer, would make these claims since they have nothing to gain from charger manufacturing. So how the battery is charged does make a difference?
Furthermore, I have been much happier with my external system than the internal one it replaced. Not only does the present system develop far more amps, it also compensates for temperature - an important safety feature. You might call it splitting hairs, I call it insurance. Lastly, where my present system uses 100% of capacity to bulk charge the batteries, my old internally-regulated 40A OEM Perkins amp bulk charged (at most) 27A during WTO. This is not to say that more sophisticated internal regulators than those used by Perkins do not exist.
As for battery costs, I agree that you can get cheap batteries at places like Wal-Mart. What I attempted to do with my cost model was to evaluate such batteries within the context of a normal charging system. With our boat, even cheap batteries turned out to be quite expensive w/respect to AGMs due to their lower charging efficiency and higher maintenance requirements. Basically, more charging translates into more engine wear, gas costs, etc. Don''t just consider initial purchase cost if you don''t have "free" shore power!
Naturally, the results depend on usage, other extras such as solar panels. However, when I see a 20% difference in life-cycle cost between my "premium" AGM battery system and a golf-cart system of comparable capacity, I take note. The inputs into the model in terms of cycle life come from manufacturers who sent me data plots. However, I cross-checked the data with sites such as wind-sun who have nothing to gain from promoting one brand of battery over another. In fact, windsun prefers premium flooded cells for most fixed installations due to longer life than VRLAs. However, these installations do not move, replacement batteries can only be shipped by land (AGMs can be shipped via air).
Furthermore, I do not think that weight in itself shows suitability - see that starter batteries can be heavy yet remain completely unsuitable for deep cycling applications. Premium deep cycle batteries have thicker plates and a lower Ah capacity than their cheaper "deep-cycle" or counterparts. The charts that Rolls sent me indicated a direct correlation between cycle life and Pb thickness - 3,000 cycles for the CS series, 1,020 for the 0.15" thick CH series. Just a few tenths of an inch for the plates and better internal support make the difference.
And Rolls stands behind its batteries with multiple year warranties. Thus, they appeal to long-term users who can take advantage of the long life. 6V Golf cart or scrubber batteries certainly offer a big bang for the buck and wide availability but I doubt that the cell grids in them will last for multiple years like those of batteries with multi-year warranties. This was one area of preference for me with AGMs - since the mat inside holds the grids firmly in place they are not as prone to failure due to vibration as flooded cells.
Please note: I was semi-sold on AGMs as a replacement to flooded cells before I started researching. That could have made my research biased. I simply like a number of features such as no gassing (under normal circumstances), no required safety gear for battery maintenance (face shield, gauntlets, apron, etc.), much less corrosion in battery compartment (venting), much higher discharge capacity (we use deep cycle AGMs to start our diesel - only one battery type on board), less charge time (due to higher conversion efficiency), etc. Naturally, I was even happier when the life-cycle cost premium of AGM over all flooded cells turned out to be negative for us. But, your mileage may vary, of course.