This is a really gray area. I understand the concerns of theme developers, but there is a big difference between outright theft and artistic or professional collaboration, regardless of the letter of the law.

I have received copies of commercial themes from clients or colleagues that I have worked with on projects. They may send me a theme to customize, and then I'd send it back. I'm not exactly using that theme myself on a live site, or maybe even keeping it. But of course multiple copies end up existing in email archives.

An even bigger problem arises with when collaborating with another designer halfway across the continent. They upload a commercial theme, I download it, make some changes, backup the changes, and upload it. They download that copy, change it, back it up, upload it again. Do that a few dozen times over the course of a project, and before you know it, there are hundreds of copies of the theme lurking around even though it is only installed once, or maybe a sandbox and a live version.

In my opinion, if the copies of the theme are being used strictly for development, it's a non-issue. However, if the copies are being used on live sites, then that's a pretty nasty thing to do.

But for better or worse, when you're dealing with digital products, you have to expect a certain amount of loss in your business model, it's inevitable, just ask the record companies!