We've had to fry our failed HDDs because otherwise the vendor would replace them with those we've returned previously. Then they would not endure RAID rebuild and the cycle would repeat.
IANAL but wikipedia says that you just need some sort of gain, not necessarily financial benefit. Warranty service entitles you to replacement only if it's broken, not when you feel like it. In that aspect you are gaining, since you're potentially getting a replacement you're not entitled to.