I think you misunderstand their point. PostIdent would only be useful AFTER someone took the time to rate the game. Steam does not require any official content/maturity rating in their store, just some subjective content descriptors. To do so would pass an additional cost onto developers. The US-based ESRB process, for example, can cost hundreds or thousands of dollars to rate a title.

Further to your point, I try to limit the number of times I provide my personal ID online. It's one thing when you show your ID at a bar and the bartender gives it back to you after a glance. It's another when I'm sending a photocopy over the internet and trusting a remote, distant party to use the data once and discard it. Even worse if they save it for future use and risk leaking it later.