Yes, if I was building a new PC at the moment I'd want 2 things PCI Express and SATA, I wouldn't necessarily use either of them at the moment but I would in the future so it makes sense to get them now.
a) Looking to future proof your MOBO in a big way.
b) Are a die hard gaming nut who absolutely has to have the next best thing.
If you just use a PC for word processing and web surfing you don't really need it and you have spotted the problem. "Available and at about the same price" at the moment PCI express is expensive, though the signs are good that it should be competitive before too long.