CardinalWolseley suggested this feature on IRC after being slightly confused about whether certain edits had been made properly.

I feel this is a very reasonable request, as especially when having multiple, smaller open edits on an entity, getting an idea of how it's going to end up can sometimes be pretty hard.

However one argument against this could be that it might cause even more confusion, especially when trying to make additional edits. The preview mode should thus probably make it very clear, that it's read-only with editing being disabled.