I would agree with your statements on most 32/64-bit systems but not on x86-64. Longmode has quite a few performance related enhancements compared to 32-bit mode, for instance:

• Twice as many GPRs and SSE registers
• FP operations use SSE by default (you can’t use SSE unconditionally in x86 code if you have to keep backward compatibility)
• Better calling convention (most parameters passed in registers)

Note that none of these improvements are really advantages of 64-bitness, they just happen to be available only in x86-64’s longmode.

The disadvantages of 64-bit systems (larger code size, higher memory footprint) are negligible on x86-64. I’ve done quite a few comparisons while porting HotSpot to x86-64, the overhead was pretty small in almost all cases.

On x86-64 all distributions go for a complete 64-bit userland with a 32-bit compatibility environment due to the advantages of longmode. 64-bit mode on other architectures (e.g. ppc64) doesn’t have similar advantages, so most distributions have a mostly 32-bit userland on these.

Anyhow, I don’t care much about an x86-64 version of acrobat but I would appreciate a ppc version.