Two important things there:
1) Making a PCB for 8 FPGAs forces people to buy in multiples of 8. That's an expensive board. Wouldn't you rather have the option of buying those 8 chips one at a time?
2) If you want to do the mass thing, you go ASIC, you don't buy 250K FPGAs. When you pay for an FPGA you pay for a)performance and b)reprogrammabililty. If you want a lot of them, and you don't need part b, you're going to save by making your own ASIC. But that day has not come for bitcoin, not by a long shot.
The whole idea of a multiple FPGA units per card is to actually save money on building the cards by benefiting from bulk chip orders pricing and pass that saving to the customer, since having cards with say 10 chips would likely cost around 70% of buying 10 of the singles. If you can afford a 350+250/card starter you can easily afford an upgrade card that costs around that price. And having a 4 chip card starting around $700, $1200 for 8 and so on and forth would make more sense than buy lots of boards that give no nominal performance increase... Why pay $1000 for 800mh/s in 4 daughter cards when the BFL gets you 830~mh/s for $599 without the need to buy an additional main board to plug it in? It's very poor efficiency and not cost effective to have the performance upgrades so small, they shouldn't be less than ~500mh/s per card ideally more or you will need a warehouse to store all those little boards to get any high end performance. I am sure you can't put too many upgrade cards per motherboard either, which means you will need another $350 board to add more after a few modules upgrades, speaking of which... how many upgrade modules can your main board hold before I would have to buy another board to get more cards?