Are voices responsible for a bad script ?

No, unless they are authors. But in real life this is not that simple. When the voices get a poor translation full of mistakes and not understandable expressions, it is a professional obligation to let the client know about the issue. If that is possible, it is good to suggest an improvement of the text if this is really possible, but on many instances, clients can't decide or they just say to you: sorry this text was validated by the company so we have to stick to it. If no solution is on sight, you don't have other choice but to insist. In the case that the text can be corrected on the spot, then what we do at PrimeVoices is recording two versions, one as it should, and the other as it is. We submit both recordings to the client and warn him that further changes should be paid. In other cases, the text is fine but not convenient to do a voice-over, as it is too long or too short, or just written in a convoluted way. If matching the pictures is a must, as it happens with all videos, then we have to apply some editing skills. When we get texts in English and then we do other language versions, we insist that translations should respect the timings, but it is rare that the translator work with a screen in front and TC.

The talent of the voice is measurable in mike-hours

Yes, absolutely. More than a naturally sounding voice, the voice actor is recognized as real talent thanks to many years of reading, announcing, performing in front of a microphone. So we can correctly argue that like the quality of pilots is measured in flight hours, the quality of the voice professional can be determined in the hours of lip service before its majesty the mike. I only understood this some years after starting my career as a radio journalist. At first I didn't know how to breath properly, how to keep the concentration and how to sound convincing and motivated, even in cases that I was not prepared to buy what I was recommending to sell with my voice. At PrimeVoices we have conducted thousands of castings in our ten years of existence. At first some people presented themselves at our studios with quite an usual argument: I want you to record my voice because everybody tells me that I it sounds very good.
It is delicate to express an opinion of someone else's voice, as the voice is part of your identity, of your personality, so we don't want any offence. But sometimes you have to be straight and tell him that his voice is not trained or sounds hesitant, etc. It's easy to recognize someone with real potential, so the best we can do is encougage him and advise him some breathing exercises, reading techniques, etc.
At the end of the day, you know that those voices that are "usable" belong to those who have a proven record of hundreds even thousands of hours. They are the right people for voice-overs and specially tricky dubbings and lip-synchs as you know that they are good, as they are quick and critical with their job, so you save studio time and you know that what you are giving the client is good quality.