I think that it looks sloppy, and when grandma(or anyone else) comes over to visit and wants to see the kids rooms I don't want to have a sloppy mess all over the walls. I currently work as an interior designer, and all of the walls in my house are painted crazy colors. Theres nothing I hate more the plain white walls, with the exceptions of white walls that kids have scribbled all over. I think that creativity is very important, and if an older kid wanted to paint a mural or something I would definitely let them.
Once the future kids are old enough to understand the difference of why they can draw on one wall and not any others I will probably paint an entire wall with chalkboard or dry erase paint. I would have loved something like that as a teen. In fact, I might do it for myself when I move to a bigger place and can have a craft room/office because it would still be fun for me.
Painting a light color over marker is all but impossible without multiple coats of stain-blocking primer. In the place we just moved into, one of the rooms was painted super dark blue with clouds. It was a terrible paint job or we might have thought it was cool and left it. Anyways, three coats of paint later and we discovered that there was pink and black marker scribbled all over one wall, and it bleed right through all the layers of paint. It was clear as day, and we had to paint that wall all over again. We couldn't see the marker on the old dark colored walls or we would have scrubbed harder when we washed them. I was so mad, I wanted to find the kids that used to use the room and make them re-paint it.