i always thought a bonus for being idle was silly to begin with. But i'm guessing the idea is that if you want to use it, ou have to actually stop building things. not just build everything there is to do, then idle. if necessary, use gold to buy things that are needed instead of building them ?

I like the "when idle" mechanic quite a lot. It is indeed an interesting choice. Should I keep developing city to get long term advantage or should I go for short term gain and get that tech quickly?