I'm no scientist, but my understanding of the way the body's energy model works runs thus:
When the body's demand for energy first begins, aerobic respiration (through oxidation of ATP via the Krebs cycle) doesn't occur fast enough to meet the initial spike in fuel demand. I liken aerobic respiration through substrate-level phosphorlyation to an engine that is powerful and efficient, yet slow to start- aerobic respiration, in keeping with my metaphor, takes a long time to go from "0 to 60" (reach full production capacity.
So, because the initial demand for energy cannot be met by the efficient, powerful aerobic respiration, the body has two other primary methods of getting energy to the cells.
The first uses creatine phosphate as a fuel, and while the breakdown of creatine for energy results in no waste products, the body also uses its supply fairly quickly, and so after 10-15 seconds or so, the readily-usable supply is gone.
The second, which "kicks in" after the creatine is exhausted but before the body can primarily utilize aerobic respiration; anaerobic glycosis. Utilizing an electron transport chain, pyruvate is producted. Pryruvate is then converted (by another anerobic process) to lactate, which in turn is used by the body as fuel.
I was under the impression the pH levels of the body lowered as a result of acidosis caused not by lactate but by the increased hydrolysis of ATP molecules, which occurs after lactate is used as fuel.
So, if you looked at the percentage of energy contributions of the body's different systems during exercise, you'd see that the LONGER the duration, the MORE aerobic something was.
something like (I'm making the exact numbers up)
100 meters- 99% anaerobic energy, 1% aerobic
400 meters- 70% anaerobic, 30% aerobic
mile- 50% anaerobic, 50% aerobic
marathon: 1% anaerobic, 99% aerobic
So if you looked at the contributions of each kind on a graph, you'd see anaerobic contributions falling off as the duration of exercise increased and the aerobic contributions increase as the duration increases.
There is no lactic acid in the human body, only lactate, which is, as you pointed out, a vital and reusable (to a point) fuel. However, the more lactate in the tissues, the more hydrolysis of ATP occurs, and as a result of that hydrolysis, more lowering of the body's pH level.
So, technically, as I understood cellular respiration, no, lactate itself does not cause acidosis. However, the process by which lactate is utilized by the body as fuel does indirectly lead to the lowering of the body's pH level.
Looking at it that way, its pretty obvious why many pioneering exercise physiologists drew the conclusion that "as lactate levels become very high, a lowering of the body's pH occurs and ability to perform decline sharply, at least until lactate levels in the tissues go back down again."
Was this decline in performance correctly associated with the presence of increased amounts of lactate in the tissues? No, but it wasn't totally incorrect either.
So while a slight change in terminology should be advocated (saying lactate instead of lactic acid, no more referring to "oxygen debt" etc, etc) it's not like the whole model needs to be thrown out as useless. More importantly, it doesn't change how to go about training, just how to correctly describe what you're doing.
I got your backs, Lydiardite cultists. :)