There are a few very select situations where the carb tabs can be helpful - outside of these, the drawbacks make them not worthwhile.
#1 - Beginners who are not comfortable with measuring for priming a whole batch at a time or simply don't have the extra bottling bucket to do so.
#2 - Experimenting with the amount of carbonation to add (i.e. you can add different amounts to different bottles).
#3 - If you keg most of your batch (and carbonate with a CO2 system), but want to put a little bit in bottles and don't want to calculate your priming sugar or deal with per-bottle dosing (messy).
#4 - If you have a bunch on hand from kits, etc. and just want to use them up (although I'd just toss 'em in the boil for a batch to get rid of them).
Drawbacks:
* The mixture of DME, dextrose, head enhancers, and binders often does not fully get eaten by the yeast during bottle conditioning and can leave visible "floaters" in your bottle. They are fine, but look nasty (especially to your friends who don't brew!).
* Carb tabs take longer to carbonate in the bottle than batch-priming does (it needs to dissolve, then get eaten, etc.)...can add as much as a week or two to your bottle conditioning.
* I used them on my first few batches and ever since I switched to batch-priming, my beers are clearer and the flavors are crisper (although some of that may just be from my brewing getting better).
* By handling each tablet and physically touching each bottle top, your odds of an infected/contaminated bottle are DRASTICALLY increased.
* They are really best for 12oz and smaller bottles. I've heard MANY MANY complaints about tabs not properly carbonating in 22oz bombers and in growlers.
CONCLUSION:
If you've got them and want to try them, go for it and see how they work for you. If you've got the bottling bucket and dextrose, that's certainly the better route.
You might even want to split a batch out and see for yourself the comparison. A standard carb. for a 5gal. batch is 3/4c of dextrose. That's 1.2oz/gallon. Bottle 1 gallon of beer with tabs. Then add 4.8oz of dextrose to the remaining 4 gallons and bottle that. Then wait and taste-test for yourself!