Depends on the mids.
Most are designed to operate in an infinite baffle alignment, which means they would perform optimally with no enclosure (i.e. keep them open in the door). However, you still want to completely isolate the front wave from the back wave of the speaker, aswell as solidify the mounting panel and location....which means you want to completely seal the inner door with something relatively strong..such as a few layers of sound deadener, and solidly mount the mids and use something such as molding clay to make it a resonant-free mounting surface.
There are a minority of car audio mids that would be better suited for an enclosure. Likewise, if you are using home audio drivers in your car (such as most people using a DIY comp set are doing), then it may be beneficial to use an enclosure.