I have a personal view on this.
If I was to get an OEM version of Windows, and I knowingly reinstalled it on other motherboards simply because Im a casual upgrader, then I would disapprove. The retail versions are there for casual upgraders who will knowingly upgrade regularly, and from the outset it makes sense to get the retail version.
Now... lets say I built a PC for my dad. And the motherboard failed, or was problematic. The chances are that I probably wont want that brand of motherboard again, and will get an alternative motherboard. I will probably reuse the OEM license too. Yes its technically illegal, but for the sake of a supplier that supplied me with a dodgy motherboard, Im not going to shell out for another OEM license. I dont recommend it, and Ive not ever done it, but Im just giving my opinion.
To then compare me to someone who is a genuine thief (ie someone who installs a cracked/pirate Windows and has never paid for the privelige of using the software) is downright silly.