Yes. This is their main voluntary use.if you have Sistine Chapel, you can easily pop borders without being in Caste System.
but I was initially going to append something along the lines as "...or perhaps I just suck and do not know how to use them"
Beakers in representation but not caste system seems the most generally useful to me. I also use them quite often to get things going in ice/tundra fishing cities, but those tend to be crappy sites that I suspect I shouldn't have settled anyway so I'd be cautious about offering it as a tip. It's possible the tip should really be "don't settle any city where 1 hammer from a citizen is an attractive proposition"! Moai city excepted of course, especially if stone doubles it to 2.
I've also used citizens to deliberately not get GPP for a few turns so a GP pops somewhere else first. Which is using them precisely because, as the OP said, they suck.
Population storage when no useful tiles are available surely doesn't count as using the citizen specialist by choice does it?
What a great way to post a question. You claim that citizens suck, yet not being privy to the fact YOU must suck as well, for not having taken advantage of them in half a dozen different situations that you no doubt failed to take advantage of...
Hmm....
There are simply too many reasons to count, but here's a clue. They get all bonuses other specialist boosters get AND they give 1 h. I'll bet 100-to-1 that you're primarily a cottage spammer, and hence haven't moved up to something better like SE yet. If you did, you should have known!
Also, you should consider the globe as well for buildings worth building use citizens. If you get to drama late or aren't in HR then you may not be able to use the whip to build it and high food feeds a lot of crappy citizens.