Well I can think of three at the moment:
a) Not as noisy as fuselage mounted engines.
b) More space! Would be pretty interesting to see the engines of the 777-300ER mounted on the fuselage
c) If more than 2 engines are mounted side-by-side on the fuselage (ie 4 engines in total), a fire could spread more easily than on the wing.
Disadvante would be the yaw effect if an engine failed on one side.
Regards,
Mark