As exhaustively said before by many, 99.99% of the market will NEVER open a computer case, let alone add a PCI board, which apart from hardcore gamers who need new bragging rights every quarter, is simply unknown and useless to any normal user. Apple is the ONLY company realizing that, and that's why it's so successful, growing more than double the ordinary IT sector.
I've never considered adding anything more than RAM, an external drive or assorted USB or FW devices...just as pretty much anyone out there...add-on boards had only two roles in the Mac's history...either to run PC software or to run Apple II software in the Mac LC...