Like I said, I'm in favor of requiring travellers to be fully immunized, but it's actually not really possible to stop *all* imported cases unless you're willing to put all incoming people in quarantine for a couple of weeks, whether they've been immunized or not.
90% isn't really good enough when we're talking about measles, which is about the most contagious of the serious diseases. You have a good point about the PBEs being of little significance to increasing immunization rates overall, but that doesn't mean it's not worth doing (see below).
Given my difficulties getting my own kid fully immunized (which I finally succeed in doing), I think we can boost immunization rates far higher simply by making it easier for people to get their shots than going after PBEs. Free clinics operated out of schools, well advertised, would be a good way, as well as permanently open walk-in immunization clinics.
Arguably, then, it's misplaced zeal to go after PBEs at all rather than just improving immunization availability. However, one way in which PBEs are more dangerous than simply lower overall immunization rates is that PBEs concentrate geographically, leading to local communities with, say, 70% or lower vaccination rates. These local communities are capable of supporting an epidemic, which, in the case of measles, could lead to a lot of infections in the surrounding 90% immunized population. Suppressing these *local* fires, which are a threat to the surrounding community, is a good rationale for going after PBEs. (Remember, immunizations don't always work, so an epidemic in a localized population can lead to lots of infections in the surrounding population.)