it improves the pin connections, it's more stable, dust isn't going to be a problem, etc.
i dont belive that one bit - i have never had a CPU come out of the socket, ever and i have never heard of people's CPU's falling out of the sockets - laptops or desktops. A well applied heatsink will keep that CPU in place and that is ontop of the well designed socket for holding in the CPU, why most laptops i have seen have the heatsink screwed in. Do you not think cpu socket makers like Foxxconn dont test for this.
And why would dust be a problem in the cpu ? soldered or not, your points are completly invalid, and if they were valid, why not solder in the cpu in ALL of their systems? not just certain ones?
There is one reason why they did this, and previously did it in most of their system i recall in the older G days. - they do not want you to upgrade the CPU, instead they want you to buy an entire new system, this has always been a problem with apple, your upgrade path is always been minimal and lmited to only what Apple wants you to use or upgrade to, since Apple is in the software and OEM market, this does allow them to have greater control of their products and likely results in cheaper costs for support for people who's system died because of some poorly written 3rd party application / drive.
They have gotten much MUCH better, yes, but why solder it in, there is NO valid reason except they do not want people to upgrade (those who would know how) to change it out and get a cheap upgrade vs buying a entirely new laptop. I know it is a small market, but that %5 is %5 more market share which = considerable profit when you consider %5 of the computer market who own's a mac and wants to do their own upgrades.
Sure MS let's in so many 3rd party companies (the reason why Windows becomes unstable), and i like to have a large selection, if Windows closed it's doors to companies and ran in the model that Apple does, it would be just as stable in terms of hardware not failing and software not crashing. MS can learn alot from Apple in how to better it's policy for allowing developers to code for windows and they need to be alot more strict about it, if apple let their doors open like MS, then Apple would become windows in terms of stability and downtime.