I am under the impression that termsets were a rarely used and poorly understood concept in lojban. I think I finally understand (at least one use of) them, though, and it's all thanks to Fantine (from Les Mis).
Here are two lines from "I Dreamed a Dream"
> There was no ransom to be paid
> No song unsung, no wine untasted
the verb "to be" is elided in two places in the second line.
Similarly, the phrase "To err is human; to forgive divine" elides the verb "is".
Now, neither of these would use termsets in lojban because the lojban translations would differ by bridi rather than by sumti and so use bridi tail connection rather than termset connection, but I feel like I finally grasp the concept.
--
mu'o mi'e .arpis.