There are plenty of exceptions and special applications. Special ops uses 100 rd drum mags and the like. The biggest change since 'Nam is in the number of mags carried. Guys I know that served in the sandbox tell me that they routinely carried over a dozen M16 mags and 4-5 M9 mags. That's a lot of firepower.
Would you store magazines in a loaded condition, or would that weaken the spring?