Last point: Why do you constantly feel the need to shove words into my mouth, Manufactured pc's are more likely to use non standardised mobo sizes in pc's as they are looking to sell a pc as it is, not one that can be customised afterwards. Manufactured pc's are sold to a mainstream audience, my grandma recently bought a manufactured pc from dell, she will not even be thinking about upgrading her pc afterwards. Manufacturers know that they will also be dealing with these people and build accordingly, manufacturers who build parts know they won't have this problem.
http://www1.euro.dell.com/uk/en/home...=ukdhs1&~ck=mn
http://www1.euro.dell.com/uk/en/home...=ukdhs1&~ck=mn
http://www1.euro.dell.com/uk/en/home...=ukdhs1&~ck=mn
http://www1.euro.dell.com/uk/en/home...=ukdhs1&~ck=mn
I think its clear to see that companies like dell arn't always thinking on the lines of upgradable pc's.
In no way shape or form are those examples as best adapted to upgrading as a custom built pc that I have thought about and considered how future proof it is. Oh and besides, dell isnt even as kind as to show the mobo's form factor, great so I won't even know the mobo form factor untill I've bought it :L
And btw a big reason to want to upgrade your mobo is the cpu socket, as If i want an i7 chip that wont be possible without a new mobo :L