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I think that Roux is so against is due to the risk that too much emphasis is placed on the GAD and it is seen to have influenced both his actions and integrity as a witness.
The massive risk to Roux is his clients testimony being struck from the record if he is found to be an unreliable witness (Nel mentioned this possibility during argument for referral).
With OPs evidence being struck from the record you have no explanation from OP about what happened that night, all you have is a young woman shot dead behind the locked door of a tiny toilet cubical. If OPs testimony was struck from the record I don't know if the bail statement or evidence in chief would stand but even if it did, you would have a story full of holes with no explanation at all. Roux will be very worried about this even though the chances I feel are low, its still there and a massive, massive risk. I don't think he thought Nel would go down this route. Roux may have not seen it coming at all or thought that his interpretation of the law was 100% right and sound. It is probably the latter.
I can not get my head around the fact that the next (probably final) witness the defence have lined up is yet another about phycological state!
Writing that has made me think if a third possibility, the Roux saw that risk of referral for assessment but thought that by having the phycological witnesses as the last two he could push assessment until after the defence rested giving a delay...
Now I write that out I realise it doesn't actually make any more sense, there is no benefit, humm okay, maybe my first thought where right and it was a play to work in an appeal loop hole in the event of conviction.