Advantages of the long travel time:

• The spacecraft will arrive at the Moon with much less velocity, and so will need less propellant to reduce their speed enough to enter lunar orbit. The saved propellant will be needed later to conduct orbit-reshaping maneuvers and subsequent adjustments.
• Any volatiles onboard will have plenty of time to leak away before they would interfere with scientific measurements. The spacecraft will be rotated en route so that the Sun can warm it all over, encouraging complete outgassing before the science phase begins.
• Each spacecraft will carry a clock (an ultra-stable oscillator) that is very susceptible to temperature changes. The long cruise will provide ample time for the clock's temperature to stabilize after it is powered on.
• The extra time will enable the GRAIL team to devote its attention to one spacecraft at a time when conducting various tests and maneuvers during the travel period, and will make it much easier to arrange for one spacecraft to arrive at the Moon 25 hours after the other. That is so the DSN antennas in both Goldstone, California and Madrid, Spain will have a good view of GRAIL-A during its critical Lunar Orbit Insertion (LOI) maneuver, and then an equally good view of GRAIL-B's LOI after Earth's rotation brings the antennas back into position again.
• GRAIL's long cruise time will also make it much easier to time the arrivals so that the spacecraft can begin science operations on March 8, 2012 and take advantage of the full 82 days during which they will be able to point their antennas at each other while keeping their solar panels facing the Sun.