The comet Tempel was found by a NASA probe to have only 6% water ice on the surface. To me, a comet core just looks like an asteroid. The NASA Deep impact pages really seemed to be trying to imply that they were correct all along. See for yourself - it just sounds very suspect. They still seem to be saying that comets are made of comglomerations of dust and ice. They couldnt/didnt measure the proportions of substances identified. It does show that the comet is markedly layered (like a rock) and contained the silicates olivine and spinel also clay minerals and carbonates.
The Deep impact site explanation for the layering is "comets might be formed in layers. Imagine two small proto-comets smashing into one another, sticking together and flattening like pieces of playdough," Hmm
What would occams razor suggest if - it looks like rock. It has layers like rock. It is at least partly composed of rock (ice chambers inside are only infered and only tiny crusts of ice on surface).
Also.. It is cratered like an asteroid.. do craters look the same in ice as they do in rock?
I think a good possible explanation is that comets are mostly rock. The layers could be formed on a planet or moon etc and was blasted into space by collisions or whatever.