A weapon mounted light serves a different purpose than a hand-carried light. First, if you only have a weapon mounted light you have a tendency to use the weapon as a pointing device for the light. I carry both a weapon mounted light and a carry light. The major reason for the weapon mounted light is the strobe.
With respect to carry difficulty, see images below.
Carry light: Surefire E2D LED Defender
Weapon Mounted Light: Insight Procyon
M&P 4.5" 45, Crimson Trace LaserGrips, Insight Procyon Weapon Light
M&P 4.5" 45, Crimson Trace LaserGrips, Insight Procyon Weapon Light in SmartCarry holster with spare 14-round magazine.