I'd say sponsoring for a green card is usually not a good move for an employer, as the employee will have more freedom to change jobs once he gets the green card. Also it's a very long and costly process.
They'd have to really want to keep the employee, and to have exhausted all other visa options.
I know that some employers treat green card sponsorship similar to a bonus: if you leave before z years after getting the green card, you owe them $x0,000.
I sincerely doubt it, since the companies I've heard of that do it are large multinationals. As I point out below, it's much the same as a signing bonus or moving expenses.
They'd have to really want to keep the employee, and to have exhausted all other visa options.