It's not just "in the card", it's in the CPU (SoC or System on a Chip in ARMese). The card is essentially just a carrier/socket for the CPU/RAM, which in ARM-land is typically soldered one-on-top-of-the-other directly onto the motherboard.
The division-of-labor between motherboard and CPU is different with ARM versus x86. ARM is more integrated, mostly because this is a good thing in typical ARM applications. A lot of what would be regarded as the duty of the motherboard in x86-land moves to the CPU in ARM. An ARM motherboard is going to be mostly about providing physical connectivity to the rest of the system.
The point of the card is to enable the development of the rest of the system (which takes significant engineering effort) without being tied to the current hotness in the ARM world. From the hardware design standpoint, it lets you hedge your bets for the future.
Your point about the software platform not benefiting from the engineering decision to put the CPU/RAM on a card/carrier is certainly true. You are correct that a given, say, Linux installation will not appreciate having a different CPU stuffed in, because of all the driver issues you mention. That said, it is also the case that the bet-hedging that modularizing the CPU/RAM buys applies to the software also. If, say, the open-source Mali driver never takes off, and Intel suddenly drops a good open source PowerVR driver, you could switch from the AllWinner A10 to a TI CPU and pivot quickly. If, say, NVIDIA provides a good open source driver for their GPU, you could go that way. The ARM driver situation is kindof at a tipping point at present. It's hard to guess who's going to be the first manufacturer to provide a suitable open source environment. Whoever does, while delivering something "good enough" from a hardware standpoint (which is constantly changing), made widely available in smaller quantities, at a reasonable price, stands to gain a significant market advantage. The Allwinner A10 is very close, if they could just be made to understand the importance of getting their cedarx and Mali drivers open sourced.
Meantime, though, the work of designing tablets and notebooks and netbooks and servers and routers and other form factors can proceed. Sourcing screens and keyboards and touchpads and touchscreens and batteries and designing enclosure molds and all that stuff takes time too, and can be happening in parallel.