Let's see if I can answer without writing a 10,000 word thesis.
This is my understanding...I reserve the right to be wrong.
The installer wants and expects to assign a Swap partition during installation; therefore, if the user doesn't assign one, it will use one that already exists in the list...even though it's on a separate disk.
So, the portable won't have swap on any other computer other than the host computer that was used to make the portable version anyway.
How is the host computer affected? The first time that I made a portable version of wattOS, the host computer was running wattOS also. I allowed the installer to assign the host computer's swap that time...merely because I didn't catch it. However, there were no bad side effects...I assume it's because it was the same OS...but I'm not sure about that.
Later on...while trying to figure out my grub situation that I referred to in an earlier post, I made another portable with a Debian host computer...
again allowing it to assign the host computer's swap. On a subsequent boot of that host computer, and I can't remember the exact "fail" or "warning" message that was listed during the boot process (or the shutdown process)...but the Debian computer's swap was no longer accessible...or something like that.
So, I had a portable with no swap except on one computer, and I had one computer with no swap any longer.
So...to me, there were two options.
1. Don't allow the installer to assign a swap partition.
2. Use gparted to make two partitions on the portable USB...one being around 500 MBs in size (on an 8 GB USB) to assign as swap during the installation.
In my case, the "no swap" option is preferred as I only use the portable version on rare occasions and for short amounts of time. If I were using it more, I would want the swap partition...which is also easily done.