why virtualmin pro?

here follows my opinion.

If you use this on a home system or just for yourself or testing then GPL is fine.
If you use this software professionally and have or want paying customers then I opt for VM Pro.

It allows you to offer features to your customers not available in GPL, like (an important one) install scripts and a web page editor. There are more feature but please speed up on it by reading documentation and compare yourself.

Secondly, choosing for a Pro version proves to your customers that you are dedicated to offering a Professional product and in general will be taken more seriously by (potential) clients.
(The GPL is also professional however from a sales point of view customers like hearing professional before free as they don't know what to expect)

The webhosting market is quite ruined by those kidresellers without servers and no support while promising 1000's of everything they can't deliver.

Thirdly By buying this product you also support the developers Jamie and Joe and therefore support them, so they can put even more time (ie hire people) into developing, upgrading and answering your questions in overall it will benefit you as well.

I can go on, but people will do as they do.
This is my opinion anyways.

Ronald has covered most of the high points. I'll also point out our handy-dandy comparison chart here (I found this link cleverly hidden in plain sight in the main menu at the bottom of every page here at Virtualmin.com):

http://www.virtualmin.com/compare.html

But, we're not begging you to buy Virtualmin Professional. If you find Virtualmin GPL satisfies your requirements, we encourage you to keep using it.

The reason we have a GPL version is so that a lot of people will use it. We want people to use our software--we aren't writing it for our health or our amusement, we're writing it because we want people (a LOT of people) to use it. If Virtualmin GPL is all you can afford, or all you need, or the only thing that satisfies your need for FOSS software, then by all means use Virtualmin GPL. It's the most powerful FOSS control panel available (actually, it's also more powerful than almost all of the proprietary control panels, as well).

So, use Virtualmin. If you need or want the features of Professional, upgrade. If you don't, don't upgrade. We'll like you either way. (We can't afford to jump through quite the same hoops we do for paying users...but we still like and appreciate that you're using our software.)