I just looked it up to see what the lifespan of the dry erase markers might be. According to Expo's website, a tightly capped marker stored horizontally should last for 2-3 years.
I'm pretty sure that my Skilcraft US Government grease pencils are at least 35 years old and they still work. Granted, I don't use them that much, but try getting any other type of marker to last that long even just sitting around.
http://www.bscsource.com/Assets/Imag.../400/69421.JPG
And one other advantage is that they will write underwater.
I probably should think about trying to find some refills for them. :)