a) no the haste cap is not enough to make the talent unnecessary
b) no the talent plus additional haste does not take you below 1 sec gcd, because 1 sec is the GCD cap.
You would need somewhere around 5500 haste rating (and the 5% haste raid buff) to reach a 1.0 sec GCD without the swift rejuv talent.
Since you aren't really losing much in choosing swift rejuv over the other talent options, just take it and have the guarenteed 1 sec GCD for Rejuv. Being able to respond faster after casting a rejuv can be lifesaving. To say nothing of being able to toss multiple rejuvs out faster for raid healing.
And in a matter of months it wont matter as the prepatch for MoP wont have the same talent trees anymore.