My employer doesn't do that -- it uses a stick rather than a carrot in that it charges an extra $100 per month to cover a spouse if the spouse is covered by health insurance at their employer. Depending on the difference in the plans, that could be a small price to pay.
What you describe seems more than a bit unfair to employees who don't have spouses who have family health coverage available. Those are likely to be the people who earn less money, too, as they probably have a stay at home spouse or a spouse with a lower-paying or part-time job.
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)