daveman692 just showed me the best use of it yet:
/editpics.bml.error.toomanykeywords=The [[?numwords|keyword|keywords]] "[[words]]" [[?numwords|has|have]] not been saved as [[?numwords|its|their]] user picture icon already has [[max]] keywords.
The parameters here are "numwords", "words", and "max". BML then takes the current language, sees the [[?...|...|..]] construct, runs the first parameter (numwords) through that language's plural function, and takes the n-th phrase.
I've posted about this before.