Why is Rogue Trader the only one of the games where Frenzy requires you to make an All Out Attack.

And if you're always forced to All Out Attack, what purpose does Battle Rage serve since you'll never have a reaction to parry with anyway?

Actually, it's the same in DH. (Check the errata).

As to your second question, I would assume that the Battle rage would allow you to go on All Out Attack (for Frenzy) yet still keep a reaction for parrying only.

Pretty awesome combination.

Battle Rage does not let you have a reaction to Parry though. It just says you can Parry.

This would be for some corner case when you somehow aren't eligible to make an All Out Attack. You'd still be unable to Parry nearly every round.

Personally I allow the use of a Parry action even if you've All-Out Attacked; otherwise the talent is effectively useless.

I wrote "assume" because it wasn't really explicitly stated in the Talent's description and like Errant said: the Talent would be rather underpowered if you couldn't get that parry with more regularity.