There was a time I felt the same way. But with 3,100+ free diamonds with nothing to spend them on currently, sometimes it's worth spending 10 for the 5th turn to preserve medals, which I can spend on expansions. As long as I keep enough to drop my next diamond expansions when I age up in a few weeks, I can spend a few here and there.
Even though I know it is all in my head, I also think spending a few on an extra turn now and again helps 'prime the pump.' Again, all in my head, but it helps me justify dropping a few now and again.
Having said that, I will never spend diamonds on an extra turn unless I'm 100% certain it will successfully complete the negotiation. Like all negotiations, sometimes you've got to give to get.