I think Arthur's comments pretty much summed up my opinion on the subject as well. "Back in the day" the preferred shooting style was a high wrist grip with a straight arm but that seems to have changed. I will leave it up to folks like Arthur to supply the reasons though I have no doubt that bow design changes were a big part of it.

I personally shoot with a slightly bent bow arm as I believe it promotes more control and a steadier aim. That is typically the form style I pass onto others.