One of these resource developments is enough to supply your entire civilization, so long as there is a traceable trade route (river/road/sea) to your capital. However, reasons for developing additional ones include:
1. Having another one to trade to other civs.
2. Having one in case nasty barbarians/enemies pillage your original resource or take your city.
3. Contributing to the hammers/food/commerce in other cities by improving the resource squares there.
I would never trade any of those 'nessesary' resourses to another civ, unless it's obsolete (like copper after axes) )
what can be very confusing is that Physics reveals Uranium on the map, but (at least in 2.08 warlords) you don't get it as a resource in your cities until you research fission. it is enabled by a tech completely different from the techs that let you see it (physics) and work it (mining), odd concept.
what can be very confusing is that Physics reveals Uranium on the map, but (at least in 2.08 warlords) you don't get it as a resource in your cities until you research fission...
Well, historically uranium was used for various purposes before they figured out fission -- for example to make really pretty colours in fancy glasswares. Which may help to justify its value as a bonus resource before it gains strategic value.