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I'm guessing Joe Montana would never have made it there unless he threw a shit ton of practice thows, though.Ignored
The fact is that Montana, and every other professional athlete, did play in games. The practice was a supplement to the games which they were engaged in since childhood. In essence, they've been "live" all their lives. The practice you speak of is aimed at developing skills, such as reading defenses or devoloping timing on passing routes. It is NOT a substitute for the actual games.
The practice in trading is not on a demo, but in analysis and development, both of trading systems and personally. Trading on a demo is NOT the same as trading live. Anyone who's done both knows this. Demos serve a very limited purpose and as long as one understands this, and does not delude themselves, then they're fine.