> You could develop software for customer#1 and use the same software for customer#2 with a non-Copyleft license too.
No, I cannot. Work done for hire is owned by the employer, not the employee. But, if employer chose GPL-covered software, e.g. Linux kernel, then he is forced to release code under GPL, so I can develop my code, e.g. kernel module for customer #1 and then use it with customer#2, because GPL protects my freedom. I don't need to reinvent the wheel, like with proprietary code.
If the employer chooses any open-source license whatsoever then you can use the code with the next customer (assuming the next customer is ok with the license). There's nothing special about Copyleft licenses there. Well, except for the fact that a Copyleft license is far more likely to have customer#2 say "no you can't use that, it's GPL, are you crazy?"
No, I cannot. Work done for hire is owned by the employer, not the employee. But, if employer chose GPL-covered software, e.g. Linux kernel, then he is forced to release code under GPL, so I can develop my code, e.g. kernel module for customer #1 and then use it with customer#2, because GPL protects my freedom. I don't need to reinvent the wheel, like with proprietary code.