That sounds like a pretty good compromise, but you can imagine that it will still cause permanent employment friction. For example, say you run a small contracting company with a few 3-month probationary employees on staff. You're waiting to hear about the next big job. When that 3-month period gets close and you still haven't got word on the new job, you might be tempted to let them go because if the job doesn't come in, you'll be paying their salaries without the equivalent revenue coming in. That could break your small company. If the job does come in, you'll need to scramble to replace them and push your permanent staff harder, but you'll still be in business. If you weren't bound by a three-month requirement, you might have keep the probationary employees on staff a bit longer waiting for the job to come in.
The real-world isn't this cut-and-dried, but to a lesser extent, these are the tradeoffs that the employer has to make every day.
Most companies specify a 6 month probation period where the contract can be terminated with 1 month notice. That should be enough to plan ahead.
The customary 3 month notice only applies to "permanent" employees, but the company is free to specify a 1 month notice for those employed for less than 5 years if they are not confident.
The real-world isn't this cut-and-dried, but to a lesser extent, these are the tradeoffs that the employer has to make every day.