Just in case someone stumbles across this article:

The formulas have changed, so that it is now 20% faster to renew a creep than to spawn a new one. The costs are the same. See http://support.screeps.com/hc/en-us/articles/205990342-StructureSpawn#renewCreep 

I still think it is not worth it (except for special cases as discussed in the thread of neomatrix's link):

• 20% faster may just compensate for the lost time of traveling back to the spawn point.
• Another thing to take into account: Requirements for the best configuration of creeps change over time (tend to get bigger of course). So, it is maybe better to just let them die and build new ones with the currently best configuration.