I'm sorry, but paying someone to take off for more than 1/3 of the year? Is this based on how long they are with the company before said leave? 4.5 months of maternity leave is very generous, and honestly enough for a physical recovery and to spend time with the baby. I'd even be in favor of 4-8 weeks maternity leave for new dads.
I'm all for maternity leave, I think it's important, and there are too many companies that don't offer any, or enough. That said, there's a point where you really aren't an employee anymore. I'm not saying they should shorten their leave, but making it longer probably isn't a great idea.