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For example, I was annoyed when I checked my trophies Tab to see that all my details from a previous act were gone!! Surely this was unnecessary!!

Corwin, I believe it was a matter of how the lists were coded - as Lar wrote => bug fixing was easier by dividing the acts strictly. Guess, this goes for the entries as well.
Kiya

@Morbo: I know act/chapter design from adventure games => and it is common there, not to return. Advantage of linearity is not having missed something vital - disadvantage is not being able to clean up loose ends.

I think, if BD had not been designed in this linear actlike way, it would have been a terrible way to find out, I need an item from Act 1 to solve a major quest in Act 4 <shudder>
Kiya