Because most things simply won't run if you don't have enough memory. They will run with an underpowered CPU, albeit a bit slower.
PCI-e is not a solution here, access times are still calculated in milliseconds, whereas RAM access times are calculated in nanoseconds. The difference in magnitude is times 1000000 (one million). OK, this does not paint an entirely correct picture, but still, real-world access speeds are 100-200 times faster for RAM vs. disk. So no, you can't make up for insufficient RAM with swap. It just won't work, since everything becomes hundreds of times slower if they should work at all.
It's not the fact that PCI-e ssds are 100x slower than RAM, which I agree. It's how much swap you'd think you would actually use. Yes, 8gb ram might be too small, what about 10? 12? Mavericks wouldn't create that much swap and so the amount of data transferred between ssd and RAM is minimal and the time added is probably very comparable to the difference between a low tiered quad core and a faster quad core.