Disadvantages: You are barely hitting any of the supporting muscles - Tris, Lats, Shoulders, your even limiting the power that your legs put into it. You also have a restricted motion. Normal bench, you would bring the bar to over your chest, and when you push it up, its ends up a bit higher than your chest.
Advantages: Having the whole gym look at your for using the smith machine for benching (Not necessarily a good thing in this case).
Smith machine for bench? Perhaps, JUST perhaps, if you have some injury. Otherwise I dont see a point in it at all.
If you cant lift heavy its because of the stabilizing muscles, not your chest. Work on those muscle groups and your bench will go up with it.