You are not the only one who wonders why Boeing took that risk for the benefit of a few kilogramms....
Ambition. The 787 was a very ambitious project, on so many levels. And the sum of all the few kilograms here and there adds upp. Evolution in technology is mostly a continous, incremental process.
It's easy for us who have the benefit of 20/20 hindsight to know which kilograms might better not have been saved. Airbus has also planned to use lithium batteries on the A350, and they were not highly controversial before the recent incidents.
It's a bit ironic that a battery which is only there as a redundant contingency measure, to add to safety, now instead causes trouble, but I applaud Boeing for being ambitious- and I applaud the Japanese carriers for temporarily grounding their planes, and the FAA for issuing an AD. Every now and then there will be idiosyncratic teething problems with new technology, but no one has been physically harmed, aviation safety is better than ever, and I'm confident the issues will be resolved. Technology will continue to evolve.