It's not as bad as previous poster states, but it's not quite as simple as you make it seem either. A green card holder forfeits their residency if they leave the US for "more than 6 months", or if border patrol people feel like they've abandoned their residency for any reason. This doesn't affect most employees, but if you're a consultant working on-site in another country for extended periods, or simply travel often, you have to do way more work to get everything cleared. And even then, there's no guarantee you won't run into problems.