So what happens if say the MLB or display assy craps out on you? With the unibody MBPs, they designed the units "upside down" compared to pretty much every other laptop out there, and the display assy is designed to go on at a very specific angle for which AASPs have a special anti-static foam positioning block.
Swapping out a HDD or RAM is one thing, but have you ever done a full MLB replacement? Ever replaced the display assy on a unibody unit? It was enough of a PITA on the older style units, but without that positioning block, getting the display assy on/off would involve some rather risky maneuvering.
Not to mention, if you buy a MLB or display assy off someone on ebay or craigslist, who knows if they had any clue what they were doing when they removed it, so as to avoid ESD damage. For all you know it could be some ham-fisted moron who just grabs the thing anywhere s/he pleases. Then there's the fact that any third party board you buy will almost certainly have a SN burned into the EEPROM, so the OS will read that and report the wrong SN. You take it in for service, there's a decent chance a tech would notice the difference and then have Apple's people look up the original BOM for your unit, find out that the MLB SN doesn't match, and then come to the logical conclusion that you have done a major unauthorized repair on the unit and tell you they want nothing to do with it, kindly go away. Unless you want to pay full price to have the MLB replaced, which usually ends up costing more than the unit at retail.
If you could be guaranteed that only the HDD or RAM would fail, I'd be right there with you. Even the optical drive, though it's a little harder to get out. However, that's not a promise that can be made.