What do you mean by "making more use of the available RAM". Having a 64bit OS allows you to address more RAM, but that only comes into play for RAM beyond the 4GB threshold (32bits can address 4,294,967,296 bytes = 4GB). The iPhone 5s won't have more than 4GB of RAM (probably 2GB), so there is absolutely no benefit to be had from a 64bit OS in that regard. None. Zero. Zilch. Nada.
Edit: I just spent a few minutes looking at how all the mainstream tech sites are parroting Apple's marketing. The amount of misinformation being spread is nothing short of astounding, even for mainstream tech site standards. Statements like: "In desktop computing, a 64-bit processor can perform tasks at double the speed of a 32-bit one as it can address more volatile memory or RAM." are complete garbage and utterly wrong, but it's almost the statement of the day.
This is one of the few more knowledgeable takes:
iPhone 5S: The 64-bit A7 chip is marketing fluff and won’t improve performance | ExtremeTech