OK, found it (re-asked the person who knows for sure):
If you disable virtual memory, the system's MMU is disabled. When that happens:
- Every program you launch needs to be 100% loaded from disk when you launch that program, which could cause program launches to take longer, particularly applications which are themselves large. The impact of this will vary based on the speed of the disk or network volume the application file resides on
- The other thing that will happen is all applications will instantly take up their maximum possible RAM allocation, which even in 768, if you're running 9-era stuff, especially anything creative, could have a big impact.
In newer versions of 8 and 9, you can look at Get Info for any given application to view what an app will take when launched. I
believe (but can't confirm at the moment) that it'll show you what the allocation will be.
These penalties are there and as such I personally recommend against ever disabling VM in Classic Mac OS on PPC. (these limitations do not apply to 68k) In a system with, as you say, enough RAM, you'll never actually hit the disk when paging anyway.
Now, again, if you're largely single-tasking or running older 7-era software, have a lot of RAM, and you choose a faster disk option (like a SATA SSD connected to a SATA card) then the impact might not be that bad. As far as the speed of launching applications go, that'll depend on what type of connection you use and how good the media you get is and how fast the bus you connect it to is. I.e. if you plan on disabling VM, using the IDE bus (or the built-in SCSI bus) is the worst case scenario.
So, that's what that is.
The other-other thing to consider is that in situations involving "real" SSD media (mSATA/m.2 SSDs, SATA SSDs, certain very high end SD cards) wear leveling and other technology is good enough that the risk of damaging the drive from even fairly heavy swapping activity (for example: OS X with low RAM) is very very minimal.
But the good news, because nothing is entirely infallible, is that just because you got an SSD doesn't mean you stopped running backups, amirite?