The softly pressed bats i have owned the fibres open up sooner.
This statement is full of a bunch of oddities.
When you knock a bat in, you effectively press it further to create a "stiff crust", as Dave says. Therefore a softly pressed bat takes longer to make this happen. A softly pressed bat also really doesn't work very well. Believe me, Buzz and I received an interesting bat with almost zero pressing. It didn't work. We took it to Gray Nics to press it. They were shocked at how unpressed it was. It really had no performance and the bashing made huge dents in the face.
When you knock a bat in, you compress the face. The effect of the tiny cracks down the grain is caused by compression in some areas effectively pulling the wood surface apart. Apparently it indicates a bat that will perform well. I guess it's almost like a minor level of delamination. I can't say that it is something I've ever seen in a bat that I've knocked in or has had use for a long time and I don't really believe that it is really that important....
Finally, how do you know a bat is softly pressed? Has the batmaker told you they usually apply 50psi to a bat and you've requested 40psi? Of do they normally apply a range based on the cleft and you simply have an appropriately pressed cleft that happened to require less pressure? We see a lot of statements about softly pressed blades and I'm afraid I'm a bit of a cynic relating to them.