Solid pine is stronger in certain situation. But Plywood is designed for stability of characteristics. By glueing multiple layers in different directions, the material gets kind of homogeneous, so it is less affected by humidity levels or temperature. Everyone who has an el-cheapo table of glued massive wood knows the problems of solid wood.
Other problem of pine is its softness. Not so big a problem with laminate though.
Oh, and reconsider the extending CP. Try to make one shape. Not one with a box on top.