When I was growing up, if we were staying in a regular hotel we all shared one room. This was for my mom, dad, myself, & my sister. I don't remember staying in regular hotels very often though; if we went to the beach we stayed at a friend's home, and on both WDW trips after my sister was born we stayed in larger accommodations (once in a cabin at Ft Wilderness, once at a condo offsite). Staying in one room was not so much fun as I got older - I didn't mind sharing a bed with my sister, but my dad snores LOUD! You had to fall asleep before him or else you wouldn't get any sleep, lol.
I can see how sharing one room would be difficult if you have opposite gendered kids, once they get to the age where they're not comfortable sharing a bed any more. I also think if you have younger kids who go to bed early its nice to have seperate rooms, so the adults can stay up without disturbing the little ones. So while 1 room for a family of 4 is do-able, I think it's often more comfortable to have more space.