Thank you for the reply Nicolas.
The scenario you describe is actually probably not very likely as I suspect most users will be like me, trying to upgrade an old Joomla 1.5 site to Joomla 2.5 or 3 so the old and new sites are not likely to be on the same database or even server.
To give you some insight to my problem, I did a migration from an existing Joomla 1.5 VM 1 site to Hikashop some months ago. I used jUpgrade to migrate users and contents from the old site and jUpgrade requires the old and new tables to have different prefixes. I then proceeded to import the Virtuemart data into Hikashop but to do that I had to rename the old Virtuemart tables to have the same prefix as the new site. I could only do this by doing an SQL dump, search and replace on the resultant text file and then re-import the renamed tables. That all worked fine and I have proceeded with development, but while I have been working on the new site there have been changes on the existing site (new customers and orders and some new products). I now have the problem of synchronising those changes back into the new site, as you can imagine this process has become quite complex and I am having to use a fresh install of Joomla 3.2 as a staging server and copy updated tables across, renaming the vm tables along the way. I have yet to find out how successful that will be.
It would have made my task easier at one point if the Virtuemart import in Hikashop had a text field which allowed me to set the table prefix (it could default to the existing prefix) and I was going to go even further by saying can we have the facility to set the ip address, database name and login credentials as well so that data can be pulled from a remote server. Particularly useful as your import routine knows what it's already imported and just adds new data.
I take your point about Joomla users having to be synchronised as well. I am already using different tool to do that so I wouldn't worry about getting Hikashop to do it (although it would be great if it did!). All it would require would be a warning notice that Joomla users need to be synchronised first.
As I say, that's my view from a real world case, I hear your argument but I know the feature would have sped the process of getting my Hikashop live and made my life a lot easier!
Many thanks,
Ian