In addition to the batteries in question there are two other batteries for standby power of the flight controls. No one seems to question these?
The question is why certain types of battery have ben selected, ond how are they used. Boeing stated earlier they use the Li-ion for the main and APU bat (and can not simply replace them with NiCd) not only for weight reasons, but because they allow very high currents. So those batteries are charged and discharged with high currents. The standby batteries on the other hand are, well, in standby. They are just recharged for their internal losses, No high currents for those. Totally different usage, totally different issues.