People do this a lot. This is a well-known and old issue known as "Hidden City Ticketing" and goes by other names, as well. It is against the typical Contract of Carriage agreement passengers have their airlines, though it's hard to actually enforce: https://travel.stackexchange.com/questions/17984/is-leaving-...
Note that while this tactic can be money-saving and useful in practice, it does have certain downsides, like if you need to check luggage, or if you expect to collect your airline miles and segments (typically requires completing the full itinerary, though not always).
The big one that nobody's mentioned is that the airline will cancel any remaining flight segments on that ticket, without recourse unless you are very good at begging favors from customer service.
So don't book a round trip SFO-NYC-BTV-NYC-SFO on one ticket and skip the NYC-BTV segment, because you won't be able to rejoin either for BTV-NYC or NYC-SFO.
Note that while this tactic can be money-saving and useful in practice, it does have certain downsides, like if you need to check luggage, or if you expect to collect your airline miles and segments (typically requires completing the full itinerary, though not always).