True VTOL is great for airshows but not very useful otherwise. To take off vertically, thrust has to be greater than weight. For the Harrier, that leaves very little for fuel and virtually nothing for armament.

The operational use for Harriers and the future F-35 is Short Takeoff and Vertical Landing (STOVL). The Harrier has an interesting characteristic: for every foot of forward roll on takeoff, you generate enough lift to carry an additional gallon of fuel or the equivalent weight in ordance. Typically, they roll a few hundred feet on takeoff, adding 1-2 tons to the useful load. Since Harriers are based very close to the front lines, this makes for much more useful military operations than pure VTOL.

The Brits go one step better with their ski jump takeoffs. If you look at the British (and I think the Spanish) carriers, you'll find a ski jump. Using the jump allows the plane to take off earlier than with an ordinary short takeoff. Effectively, the ski jump doubles the length of the take off roll with a huge increase in carrying capability. I have no idea why the Marines don't use the ski jump.