Perhaps this is a little naive on my part, but it seems that Tenkan, opening the door to let the jab continue on it's way, is better harmony with the uke "boxer" than taking the punch or blocking. Physics is for every action there is an opposite and equal reaction, thus blocking a 100% punch with a 100% block is against physics. Although I am not a boxer, I believe that the jab and attack must still follow a line forward. Tenkan steps off the line and places you in the only position Uke cannot reach: Parallel or "Heiko" and facing the same way hip to hip with Uke, hence 100 % punch is met with 0% resistance. Obviously, there is more to it, but perhaps simpler things prevail best when nage is "surprised".
I don't advocate blocking or taking a punch. That being said, it would be great if one could do a nice tenkan and be in the perfect position. This will work in certain situations. In facing an experienced boxer, however, this is not always so easy because they are not always giving you the nice follow-through-committed attack. An inexperienced Aikido practitioner will try a nice tenkan and more often than not get hit with the other hand pretty quickly. There certainly are ways of dealing with this, but it isn't as easy as the theory itself.