I'm not a lawyer, but using a Pro version without paying for it doesn't sound fair.

Also, I wouldn't trust one that came from somewhere other than the official site.

Besides, the developers put out a free version, and just ask for a few bucks to help them continue developing. The Pro advantage is not that much: it's more like a little "thankyou."

So, the legal question is rather beside the point. Get Pro to show you support the development of LimeWire and the gnutella network