Hey, this is a great trip report, and I enjoyed the photos too. Reminds me a lot of my one and only white water trip ever on the Franklin. Especially the suck and pull of the whirlpools and hidden currents, even on apparently flat water, when the river is in flood. It's very disconcerting. I also agree with your mate's comment about how difficult it is to float in white water even with a full wetsuit and a life jacket on. There's not only the under tow but also a lot of air in the water.
I can also sympathise with losing paddles - but I was lucky. I lost two paddles on our Franklin trip. I didn't take a spare on that trip, but thankfully one of the other guys did and we also found a complete paddle in the trees at the Irenabyss camp site. So after losing two paddles, we were able to continue the entire rest of the trip.
I'm one of the least experienced white water rafters around, but I do think (in hindsight) that a spare paddle should be carried (depending on experience level).
I would not do a long remote white water trip without a spare paddle - I've learned my lesson the hard way. It doesn't need to be one paddle per person, but at least one per group (perhaps one per 3 people, or something like that). The chances of losing a paddle is high, and in serious white water and remote scrubby territory the possibility of being able to get out without a paddle is ~nil.
Strider wrote:if they lost a raft and kept the paddle I'm sure no one would suggest carrying a spare raft!
No, but you do carry a raft repair kit. A raft is unlikely to be lost, but can easily be damaged. So you carry a repair kit to account for the likely problems with the raft. Paddles are less likely to be damaged (although can still happen!) but are actually very easily lost - much more easily than a raft.
As I was told on my Franklin River trip (and as jmac said)...
If you get tossed out of your boat, don't let go of the paddle. Forget the raft, but don't let go of the paddle. The reasoning is that a raft is much less likely to get lost completely. It may get washed a long way downstream, but it will usually get found again (quite easily if somebody else has already gone first). At least it will float, and remain visible for a long time. A paddle, even though it may float on flat water, usually ends up going under water in any rapids or current, and often gets jammed in logs or rocks. So when you come out of a boat, the paddle is very easily lost permanently but a raft is usually found again without too much trouble. This certainly matches my (limited) experience with coming out of my raft multiple times on a flooded Franklin River.
Even having been told all this, I still lost two paddles in the one trip.
These days a spare paddle weighs ~1 kg (I bought one just a few weeks ago for rafting highland lakes - don't have a raft yet, though) and packs down quite small. That would be about 500g per person. I think it's essential enough to be worth taking even on a difficult to access trip because paddles are so easy to lose and so absolutely essential.
I guess it depends on your experience level though. For somebody like jmac, who may be very confident of not losing a paddle, it may not be necessary. But to somebody with not much experience, or otherwise not confident they can avoid losing a paddle, I reckon a spare should be carried. I can assure people that when you're freaking out half way down a huge flooded rapid and then get tossed out of your boat, and you're not experienced and confident, you don't think about holding on to your paddle - you only think about trying not to die.

It was a big surprise to me at just how easy it is to lose a paddle.
Of course, all this is in hindsight. I didn't take a spare paddle on my trip because I didn't recognise the need for it. So I certainly wouldn't blame anybody else for doing likewise. However, I would strongly recommend it to anybody who asks.

Caveat (ie, grain of salt): I'm rather unco. so perhaps it's just me?