How stable are Lissajous orbits?
Gaia's original science and technology report (see page 221, see also the summary) gives an analysis of the Lissajous orbit. From what I understand Gaia will be placed in a small amplitude Lissajous orbit, giving it an orbital radius of $\sim400000$ km away from $\sim100000$ km along the Sun-Earth axis.

In addition to the fact that this orbit is intrinsically unstable, variable radiation pressure from the sun causes stochastic destabilization of the orbit. The prediction is that once in orbit, a small velocity correction of $\sim1$ m/s is needed about once a month in order to maintain the orbit.

From a summary on Planck's orbit it seems that this applies to Planck as well.

That the SEL2 point is being 'crowded' doesn't seem like such a big problem to me. Once the satellites are turned off (both Planck and Herschel are no longer active) their orbit will rapidly destabilize, effectively removing then from the region. But more importantly, the orbital radius is comparable to the distance to the moon, this really is a tremendously large amount of space. Since these satellites are only a few metres across, the probability of a collision is likely negligible.