Not really. The letters have to be truthful, not just "positive". You can sue former employers who are being dishonest despite using "positive" language.
Sure, but you can't equally review your employers, so whatever they say will sound more authoritative than it should.
With this system, "not so good review" means "bad employee" and "good review" means "good employee".
If it was possible to equally comment on the employer, a "not so good review" from an unreliable employer would be less important than a "not so good review" from a reliable employer.