LGPL looked like the obvious choice for PCBO when the following problem cropped up - GPL Hijacking.
LGPL doesn’t spread to other parts of an application and it does allow being linked with closed-source programs. LGPL is “compatible” with GPL in the sense that you can, at your option, convert a LGPLed program to a GPL one any time you want. Such a change is irreversible. For a small project, this is a risk. If a truly skilled person adds a lot of functionality and relicenses the code to GPL, the LGPLed one would swiftly be abandoned and the GPL-changes cannot be applied back in the LGPL version.
Where lies this license problem? MPL itself has no problems with the LGPL or GPL licenses, the problem is on the GNU side. GPL has a problem with MPL.