And if that reason is you want to take advantage of a third parties cache, you can configure Unbound to forward requests onto another resolver, but also for it to fallback to doing it's own resolution if that third party resolver times out.
Unbound is easy to set up, and runs on Linux, OSX and Windows.
the caching mechanism is not there for show...
What figures are you basing that on? The TTL for the NS records for '.' is 6 days. Or are you just guessing? I think it would be perfectly feasible for everyone to run a local recursive resolver. As long as the change was gradual rather than over night.
If this were true, my life would be much easier because DNS propagation would be much more predictable than it is today. One bad server in the chain serving obnoxiously high and invalid TTLs can ruin your day. It's not very common percentage-wise, but it certainly happens every time we switch DNS over for Stack Overflow.