I think A500 is always useful for compatibility with games and the possibility to use Action Replay cartridge: don't know if A500+ is compatible with without a ROM switcher.
Some of the older 1.3 game expect chip ram to be at one memory location and fast ram to be at another low "slow" fast ram memory location. Many of these games don't work with even 1meg chip ram A500s and 1.3. Some really old game don't even work with 1.3 and need 1.2. The original old rom switchers had three dip socket one for 1.2, 1.3, and 2.04 (or 2.05 in a pinch) from back in the day, obviously the 2.04 would be switched out for 3.1 today.
The ACA500 actually help with this by providing for "slow" fast ram memory locations that is actually faster than "fast" fast ram and it works great with WHDLoad. I would love to have an A500+ with 2meg of chip a ACA 500 with very fast "slow" fast ram and an ACA 1233 with 128 meg of 32 bit ram and a 40Mhz 68030 (of course). This would blow even a stock A3000 away. Alas, if you could only get RTG graphics with the config.......
I don't know if the A500+ would have problems with a 1.3 rom. My guess (and only a guess) (someone out there knows the answer to this) is that it would not be able to address the second meg of chip ram. Can A2000s or a500 with a megachip address 2 megs of chip ram with 1.3 roms?
What about the old softbooting A3000s, are they able to address all of its chip ram in 1.3? I use to have one of these at work almost 25 years ago, but cannot remember.
I would guess that the A600 would be more incompatible with 1.3 rom particular the IDE controller and PCMCIA controller. Even the original A600 roms were incompatible with IDE controller (ie necessitating 37.300 to 37.350 roms).