Apparently my conclusion that MovableType built-in spam filters are quite good was premature. They (the built-in spam filters) seem to work quite well for sometime but have become ineffective by a new form of spam which used a single URL and left innocuous sounding messages.
As the comment by Shane for my previous post indicates, I am perhaps not alone. Whoever this Stenxxxx or Tramadolyyyy is, he or she has been very successful in defeating the built-in filters.
The end result is that I am enabling comments only for users who authenticate via TypeKey. This is certainly not the best solution but I have run out of options.