In reply to thinbennyboy: There are quite a few places in the world where it is very easy to meet other climbers, with whom may well end up becoming good freinds. I certainly have.
Marks examples are ones. Arapiles in Australia is a great place you can just turn up and meet peolple to climb with on fantastic rock at every grade. However it is geting too hot by the end of Dec and starts to cool down again in the March/April. Elsewhere in Oz it can be pretty hard to find climbing partners as a traveller. Best plan is hook up with some folks at Araps and go off to Tasmania, the Blue Mountains or wherever with them.
Paynes Ford in New Zealand used to be a great place to hook up with fellow climbers to go off on climbing trips round the South Island, easier to meet people there than at Castle Hill itself.
Meeting folks to climb with at the popular venues in Thailand is a doddle too andit makes a good sotp off on the way out or back.
If your heart is set on South America I'm not sure if such climbers meeting places are as common, if anyone knows I'd like to hear about them too.
On the other hand I know quite a few people who have been on the hot rock trips to Africa, they all really enjoyed it and made good freinds, did lots of climbing in places you probably wouldn't just go to off your own back.
On the subject of getting developing world travel expereince to become an expedition leader( like a lot of oher people, I am one)- Oz and NZ don't really fit the bill. To be honest neither does driving around in a big bus on an arranged trip. Independent developing world travel expereince will set you up much better for coping with whatever you have to deal with in your future role. So if that's your main objective then go on an adventurous journey on your own or with a mate and don't just go to western countries on a climbing trip.
You pays your money and you takes your choice. Whatever you do I'm sure you will have a great time. Hope this helps.