You're speaking of why a 64-bit system isn't needed at all, not why you wouldn't use a 64-bit browser. If someone is using a 64-bit system and has a valid need to do so, then one or two remaining 32-bit apps will indeed use more RAM for the 32-bit libs that are completely redundant save for the few apps that use them. Basically, if I have 1000 64-bit apps installed in a 64-bit system, it seems silly to keep 32-bit libraries around for one or two apps. Better to get rid of them as soon as a 64-bit version is available.
Now if you want to argue that (as you say) a 64-bit system isn't needed, that's really a separate discussion, and not one you can make without knowing the specific requirements of the user.