Many forget the intangibles a trainer provides other than "flight" instruction.
- Safety rules and protocol at the field (frequency control, pit rules, right of way, etc)
- Engine tuning and troubleshooting (better to learn on a stock 40 than possibly burn up an expensive motor)
- Trimming / CG
- Finally there is no replacing the real thing and the jitters and nerves that everyone needs to conquer. You can't just push a button and take off again after your split a wing in two.