I have been using Slackware Linux for years and wouldn't think of running another distro of Linux. I am preparing to take my hosting from the little side business it is right now to the professional world and was hoping to offer Plesk to my users in the future (hopefully the not too distant future, but that all depends on several factors).
Unfortunately my only server right now is a Slackware Linux server, so to offer Plesk I would need to have enough income to afford not only Plesk but another server, too, which of course delays my plans (I need to have more money before I can offer Plesk since I have to add a server and since I don't have much in the way of control panel options, it could be longer before I have enough customers to earn the money). If Plesk were to support Slackware I could both start offering it sooner and continue to use the OS of my choice that I am most comfortable with (and so can administer best and keep up to date and secure).
I realize from a business standpoint the Linux distros you currently support and FreeBSD (which is my OS of choice from the currently supported list for Plesk) are more important as you will probably sell hundreds if not thousands of each of those for every 1 sale you make to a Slackware user. Of course, if Plesk and others started to support Slackware, I think more people would start using it, too.
From the standpoint of how you want to be known and want to treat your customers this may not be reasonable, but in my opinion, just having Plesk tested and working on Slackware is enough. I personally would be happy with that even if there was no official support afterwards and most Slackware users would probably feel the same. I've met very few if any Slackware users that consider calling/e-mailing technical support an option... we tend to take the attitude of rtfm/google it or do without when there's a problem.
If you're really considering adding another distro, I'd suggest Debian first, than perhaps others. But wouldn't so many distros be a too big of a burden for SWsoft's staff?
Before supporting more distros, it would perhaps be more adviseable to listen & respond to your own customers over at your own forums too. There are bugs & problems posted there that haven't been resolved for months.
I dont see why offering a source package for the competent of us out there would be such a difficulty, I can see the financial standpoint coming up. Ah well, am off to install Pleask on another FBSD server