Often competion planes are required to have a canopy of specific dimensions (such as in some forms of pylon racing). Careful faring of the canopy into the fuz can reduce the drag though. In early WW2 fighters for example, the canopy was faired into the rear fuselage (eg a P-51B) for maximum efficiency. This was found to give poor visability behind in combat however so later designs sported 'bubble' canopies (eg P-51D) even though there was a drag penalty.

Besides planes without canopies look plain rubbish!!