I know that most of us actually use the Q68 in combination with a PC. In this case, the second slot is hardly needed, and the board space could be used for something else.
It would be very interesting to know how many users actually cherish the second slot! Also, if the desicion for full size SD cards was the right one.
I find the two slots a welcome luxury, maybe not an absolute necessity - The Q68 found its place on the desk directly next to its older brother, a BBQL with SGC and QL-SD. QL-SD is proof enough that you can live with one drive only, on the Q68 I do, however, like to use the second drive as a transfer drive between PC and QL worlds. Drive 2 thus is the one that normally holds a card that has a FAT12 partition on it.
Looking a bit closer, you might, however, find that on my QLs the original QL-SDs have been replaced with double slot ones from Marcel - Is that proof enough that two card slots are en vogue? (Half of the reasoning behind that replacement were the push-push type of card holders that Marcel uses, though - They feel sooo much more adult than push-pull, I might even replace my Q68 card holders just for the "click" some time)
Two card slots are nice to have for backups, and the second card is generally a welcome place to fool around without the danger of messing up the file system and ending up with a non-bootable card. Marcel's design with stacked card holders uses a lot less PCB real estate, however, and it also somewhat educates you to consider the lower slot as a fixed hard disk you should handle carefully - It's just too inconvenient to change. Not a bad thing, in my opinion.