I categorize traders into 3 levels:
1.newbie and inexperienced;
2.From experienced to consistently marginal profitable;
3.Better than consistently marginal profitable.
For level 1, you need demo account;
For level 2, you need real account;
For level 3, the two kind of accounts don't make difference.
For level 3 traders, either their minds are too strong to be impacted by emotion, or they have a solid system that eliminate human intervention, so their trading is immunized to emotion.
Therefore demo or real accounts don't make any difference for them.