Cars like Pinto's and Vegas only came with two doors. The back seats held only two and were almost worthless.
Dan
I rode many thousands of miles in the back seat of a Pinto and not only survived but eventually took the drivers test in it!

Fortunately a fair number of those miles were road trips to Disneyland and back (From Portland, OR).
I do remember the back seat being small, but I'm under 5" so it worked for me.
The back seat in the Fiesta I test drove was also tiny, but I think its more for looks than regular usage. I don't think a family of five is going to get a Fiesta to use as their family car unless the kids are pretty young.
My last car was a ZX5, so it had 4 doors. I test drove the two door before I bought it and it was definitely easier to get the kids in and out with four doors. Now that the kids are in college, I might have bought a two door hatch Fiesta if they had been available. My daughter thinks she'll convince me to let her take the Fiesta to college with her, but if I like it as much as I think I will, that will not happen!