I've done up to about 25000 vert in a week before (over ~90 miles). There's a big, big difference between occasionally running a big climb and pounding out vert every single day. Even when training specifically for steep mountain races (Speedgoat 50k), I think there's still a great need to not train over-zealously, purely on steep terrain. I've actually felt my best in the mountains closely following periods of mostly flat, faster running, with a week or so mixing in some climbing to set up the different musculature. When looking at it from the opposite direction, running flatter terrain following a period of mountain running is terrible. It takes me at least a week or two to regain the turnover to run quickly, and the way the muscles are used differently really makes things difficult. I think there's certainly some benefit to throwing a big climb in here and there, because like someone mentioned it does really rev the heart up without crushing the legs (so long as you're careful on the down), and i find trail runs with short ups and downs make great natural fartleks, but in general pounding out a thousand or more gain in a day every day for weeks takes a toll and holistically slows you down. Hopefully Sage chimes in.