Well, the big philosophical idea is that ANY EOMA-68 CPU card slots in ANY EOMA-68 machine (note that EOMA is not entirely, or even primarily about tablets -- that's just the first hardware product using it), and works. That's why Luke (aka lkcl) is quite adamant there are no "optional" features in the spec -- the only exception is for interfaces (e.g. USB, 10/100/1000-BASE-T) that can fully autonegotiate in both directions, so that there's neither a slow-machine/fast-cpu-card, nor slow-cpu-card/fast-machine case where it becomes incompatible.
yup. that's about the long and short of it. although it's at first consideration a complete pain for system designers on both sides of the interface - a nuisance for CPU Card designers because they have to substitute extra ICs such as USB-to-SATA in cases where they pick a SoC that doesn't have SATA - and bewilderment for I/O Board designers because why would they use a CPU Card in e.g. a tablet that has features they don't need such as Ethernet?? - the alternatives are absolute chaos.
the advantage: you can tell the average end-user "just buy one of these, it will work".
the alternative: think about this scenario as it is in many other standards such as Q-Seven , where you allow ethernet to be "optional" and you allow the I/O boards to "recreate" ethernet say using USB-to-Ethernet. how do you route that? well, if you think about it what you have to do is actually put down an Ethernet Hub IC on *every single I/O board*, and some sort of crazed switching, as well as put down a USB-to-Ethernet converter IC and probably a USB Hub IC as well... because the designers of the I/O board will never know if an end-user is going to plug in a CPU Card that has native Ethernet or is expecting it to be left up to the I/O Board using USB.
now expand that chaos out to SATA as well, as well as any other interfaces, and you can see immediately that a non-optional standard results in instant chaos. it's fine for Q-Seven (well... it's not. not really) where the expectation is that the Q-Seven Cards will never be removed from their carrier boards, but then why build a standard where the end-user is never expected to upgrade their system without needing a specialist degree in engineering in order to assess if the upgrade will even work?
the guiding principles behind the EOMA standards are: it must be SIMPLE, it must be OPEN, and it must work in HUGE volume.