The Earth's natural satellite has a great deal of angular momentum that seems to me could be used to derive a considerable boost for leaving the local SOI. Has any probe used a Lunar flyby as part of its flightpath? Are there reasons why this maneuver is not used regularly?
The STEREO satellites used multiple gravitational assists from the Moon to significantly decrease the amount of fuel needed to put those two satellites into heliocentric orbits. The first flyby resulted in STEREO ahead (STEREO-A) being ejected from the Earth-Moon system with a semi-major axis slightly less than that of the Earth-Moon system. STEREO-A has a period of 346 days.
STEREO behind (STEREO-B) didn't pass as close to the Moon as did STEREO-A. It didn't quite escape. It instead went into a large orbit and later re-encountered the Moon. This second flyby did eject STEREO-B from the Earth-Moon system, but with a slightly larger semi-major axis than that of the Earth-Moon system. STEREO-B has a period of 388 days.
The following nasa.gov page contains videos and graphics that depict how the two STEREO satellites used the Moon for gravitational assists:
STEREO's Routes to Solar Orbits
The Japanese Nozomi spacecraft also made use of a lunar flyby. Unfortunately, things went awry on the subsequent Earth flyby and then went awry again at Mars.
Lunar flybys can be useful if the intent is to stay in the vicinity of the Earth-Moon system. A number of near Earth vehicles have used the Moon's gravity to reduce propellant needs. The WIND and GEOTAIL spacecraft took advantage of the "double lunar swingby".
Another proposed use is to reduce fuel needed to place a vehicle in geostationary orbit. Putting a vehicle into a geostationary orbit is a surprisingly expensive operation, particularly if the launch isn't from an equatorial site. The delta-V using a traditional geostationary transfer orbit followed by a circularization and plane change burn at geosynchronous altitude is greater than escape delta-V. Instead transferring into a highly eccentric orbit that reaches the Moon's altitude at apogee enable using the Moon to affect the plane change and to raise perigee to geostationary. See Ramanan and Adimurthy, Precise Lunar Gravity Assist Transfers to Geostationary Orbits.
Excluding Nozomi, there's a common thread in these uses: The vehicles aren't going to another planet. One reason the Moon isn't used much for interplanetary missions is because things don't line up properly, and in the rare cases when they do, the opportunities are rather short-lived.
As for why it is not used regularly, it's because usually time is of the essence. If you're trying to hit a particular outgoing trajectory to, say, Mars, which is good for maybe two or three weeks once every two years, the Moon would be potentially in a useful position for a gravity assist for only a few days during that time, if at all.
You need about three weeks of launch days to have a sufficient probability of launching, and your spacecraft and launch system must accommodate that entire period. So you could not mine the benefit of a lunar assist that might be available over only a few days.
That's all if you launch directly to escape. If you plan to hang around in Earth orbit and use longer-lived propulsion systems to escape, then you could plan to use a lunar assist, so long as you assure that you get into orbit in time to take advantage of it. Nozomi did that, using lunar flybys to reduce the propellant needed to escape and inject.
Towards the end of the MER-A 21-day launch period there was a little dip in the C3 curve. It was due to distant lunar flybys on those days.
Of some interest as well is the PAS-22 spacecraft, which was intended for Geostationary orbit, had a failed rocket that put it into a very poor orbit, and recovered by flying by the moon to assist in inclination changes, allowing it to achieve orbit around the moon.
The Soviet probe Zond 5 used such a maneuver in its flightpath, circling the Moon and coming back to Earth where it splashed down in the Indian Ocean, and as a bonus the two tortoises aboard the probe survived. Cosmotortoises! I think the Luna 3 probe used such a move as well but don't take me for my words.