I was thinking the same thing about the V-->I endings. There are lots of songs that MOSTLY have I and IV chords, but usually at the end there is a V in there somewhere. That's how I hear "I'm Gonna Sit at the Welcome Table" and "This Little Light of Mine," even "Girl From the North Country"-- I don't feel like they really work without the V-I ending.
Having said that, there are obviously some songs that really, truly ARE songs with just the I and IV chords -- not many, but they are out there, and I like having a little list in my back pocket.
Thanks everyone for your suggestions!