Because there are four notes in a 7th chord, they are often more difficult to play than major chords. When using 7th chords it is common to leave out one of the notes. The most commonly omitted note is the 5th, although sometimes the 3rd or the root can be omitted. Usually the left hand will be playing the root note anyway. As long as the b7 degree is in the chord you still get the effect of a 7th chord. Here are some examples of partial 7th chords.
Listen to how good these chords sound when applied to a Blues in C.
Here are two more piano parts which make use of 7th chords.