Is A Long Paternity Leave A Good Idea?

In the U.K., there's a proposal afoot to allow either parent to get a parental allowance for most of the first year of a child's life. Currently, (if I'm reading the U.K.'s literature right) mothers get the first 6 weeks at 90 percent pay, and then can take the next 33 weeks at a state allowance (that's the equivalent of about $200 to $250 per week, depending on the exchange rate). They can transfer the allowance to fathers after 6 months. The new rules would fit more into the Marissa Mayer model of modern motherhood. If mom only wants to take a few weeks off, and dad is doing primary parent duty, why can't he collect the parental allowance right away?