For me, the semicolon is one of the most useful punctuation marks out there. Commas and periods are functional and necessary, but they're like basic building blocks, essential but devoid of any lustre. But semicolons add something.
However, it seems like their use is dying out. Time and again I see them being deprecated in use suggesting their importance is diminished. For instance, quick keyboards on my Palm (whether hardware or software) relegate them to a secondary screen for less-commonly-used symbols. They're too far into the list of punctuation on my cell phone's keyboard entry. And in everyday writing I hardly ever see people use them. Often, I see people writing sentences that should have them, but putting a comma in instead, creating a run-on sentence. Sometimes I see people putting periods in their place, creating a bunch of incomplete sentences. But by and large, I don't see them anywhere.
Semicolons used to be considered so indispensable that they were positioned in a key place on the keyboard, a prime position other punctuation-marks could only envy, and for good reason; they were used that often. Your full-sized keyboard probably still has that key in that place, but most people probably imagine it to be another "anachronism" of an outdated keyboard design that dates from typewriter days.
I wonder if someday linguists will say things like "but near the beginning of the 21st century, the semicolon began to fall out of common usage" in that section of the dictionary where they talk about the interrobang and the thorn.