Walls and Ceilings - skim coat of drywall

I have a drywall celing in my hallway that is in so-so shape. Alot of the tape joints are starting to show and some cracks due to a sagging celing. I have addressed the sag in the celing and now the drywall I think needs a skim coat of mud to make it look pretty again. I dont think just a texture will due, it needs a skim coat and then textured. My question is instead of using a knife to apply the skim coat, use a paint roller and roll on a thin layer of mud. Maybe thin out the mud with a little water and roll it on. This seems eaiser than using a knife. Has anyone ever tried this? Is this a bad idea? What about cracking?
Thanks for the replies.

Even if you do roll on your mud you will have to smooth it out with a knife. I usually don't do entire skim jobs on drywall unless the paper has been torn or I'm covering texture. You may just want to repair the seams and go from there. What is your hallway like? is it vaulted or just 8'? If it's just a regular hallway then there cant be many seams. Mostly you will be dealing with your corner joints.

I was not really planning on smoothing it out. I was thinking a paint roller I could put a thin coat of mud on eaisier than using a knife. Then after I put a skim coat on I would roll on a texture.
The hallway starts at the top of my steps and is L shaped. It does not have alot of seams probably 4 or 5. Then it also shows every nail and there is cracks and a couple of square repaired patches in it. Its pretty ugly.
In the four rooms surounding the hall I totaly gutted and hung new board but the hall wasent as bad as the rooms. In the rooms the previous owner covered up the celing with square tiles over the drywall so the drywall was pretty wasted.