This is pure speculation now, but what comes to mind:
- some apps require swap, otherwise they won't install (but this will not be the reason for implementing it)
- commercial aspect - doesn't it sound good if you get RAM + vSWAP rather than just RAM (must not be, but I think this could be since a lot of commercial vendors are "leading" the projects direction (Parallels))
But what about:
You have a total of 2GB Ram and create 2 VEs. Each gets 1GB Ram = no problems (but most hosting providers do oversell - it's only for them)
You give 1 VE 1GB and the other gets 1.5GB. Well, what happens if all memory is busy? e.g. 2.5GB RAM (500MB that is not available). It will lead to problems and I if the 1.5GB needs 1.5GB before the other, the 1st VE will only get 500MB.
But if you give 1GB each and 500MB swap for the 2nd VE - both will have the same amount if memory is full (swap will be ignored (that's what I'm not sure about, but as said, I thought it is like that). You won't have problems that way. However, if the one without swap only takes 500MB the 2nd one will have 1GB Ram + 500 swap.