Ex-Genius and ex-tech for AASP who is still ACMT certified here...
Sorry to say but you're incorrect. Apple uses a mix of new and reconditioned parts, making no attempt to differentiate between the two. Basically, if you require a part for a Mac that is only a couple months old then the service part you receive will likely be brand new. If you require a service part for a Mac that is a year or two old then it's almost guaranteed that it will be reconditioned. Technicians (both in and outside of Apple) have no way to order a guaranteed "new" part unless they just happen to order it in a short time frame following the release of a new Mac.
Is the failure rate any different between the two? In my experience (well over 1,000 repairs over 6~ years) not really. Only Apple could say for sure, since they are the ones who oversee the parts, but I've never noticed a trend in replacement part reliability. The only exception to this that comes to mind was the MacBook Pro NVIDIA REP, where the revision one boards were used until a revision two board was released to fix the issue.
Is it illegal? No, Apple's warranty (AppleCare) and service agreements (what you sign when you drop something off for repair) explicitly state that they can at their discretion use new or refurbished parts, that have been tested to ensure they function reliably, for repairs. One such excerpt from
AppleCare is here.
Reasoning behind this? Apple would need to manufacture substantially more replacement components than Macs it ships if they didn't re-use parts that had been fixed and tested out as ok. Not to mention the environmental impact that would be had if such recycling wasn't in place. Almost every major manufacturer, including automotive brands, follows this practice. Not all have quite the same level of quality control but they still reserve the right to re-use components as necessary provided they have been confirmed as good.
In addition to that - while the logic board seems like the end all fix to issues like this the fact is that software can be just as likely a culprit. This particular issue was a cat and mouse game between software fixes and hardware level fixes for several years. To the best of my knowledge (haven't been a technician for a couple years now) there's still a lot of disagreement in the Apple repair community as to what actually fixes it.
My point, don't be so quick to jump on people asking questions.
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To the OP...
The majority of repairs I've seen with this have been resolved with the logic board replacement. I had several customers who worked heavily with video (one was multiple mid-2010 15" MacBook Pros for a news channel) who I never heard from again following logic board replacements. Thats about as close to personal experience that I can offer.
I'm sure you're aware of the REP covering the
intermittent black screen issue, if your Mac is affected I would highly encourage you to contact Apple.
edit- Didn't realize you were running into kernel panics. If you do have it repaired make sure you leave explicit instructions for the technician to test it using Motion (leave detailed instructions on how to) in your OS. They might further test it by booting to your OS from another Mac to see if the issue persists. Hopefully that helps.