If the tour is just to the Volcano, no. You will spend a long time in a bus to see some steam venting from some rocks. If it is part of the all day land and sea trip, and you have never been to St. Lucia before, then that is a must do. Best way to see and do and enjoy the island in one day.
Yes I agree with Bill. I just did St. Lucia last week and took a cab out to the volcano, well actually it's a sulphur spring that has an odor of rotten eggs, and I hated the cab ride, It took 1-1/2 hours to get out there up these twisting, winding, hairpin turn roads that went up and down the rain forest mountains and then you had to do the exact same roads to get back. It took the whole day. It was just too far to go by car. If you have to see the springs than go by catamaran at least you can get some sun while your travelling there. We stopped at a little beach on the way back where a Dr. Doolittle movie was filmed and it looked so picturesque but the water was dirty for swimming. You took a little ferry (1000 foot distance over and back) It was something to do to break up the cab ride but I feel like we missed some of the best beaches on the island which I hear are on the north side. Later that night back on the ship we heard some people telling the story of the ride to Hell and back referring to the same trip we had taken.
Joe, went to St Lucia in May 2000. Loved it. I got off the ship as soon as we docked and found a wonderful cab driver, named Doobie. We took 2 people from our dinner table along and hired him for the day. He spent 6 hours with us and charged $40. We would just scream stop, and he would. He took us to many out of the places, as well as, tourist attractions. He was well informed. This was the best excursion possible. We went to the volcano, that is why I chose this island. We met a 10 yr old boy, who for $1 went up a coconut tree and brought one down, opened it, and let us sample to free milk. He also took us to a botanical garden with a water fall.
The best excursion I had on St. Lucia was a shoreline cruise that stopped for snorkelling, then went to a small, absolutely beautiful cove where Dr. Doolittle was filmed, and then sailed to the Pitons. This was one of the most visually stunning sights I've had in the Caribbean. On the return bus ride, we stopped at a great restaurant with an old grist mill, went by the sulphur springs, a botanical garden, and several other sites. By far the best part was sailing up the coast and docking at the pitons. I highly recommend this. The suphur springs are a bit anti-climatic in my opinion. A small 1/2 acre (?) of bubbling mud.
BTW: We found everyone on the island to be very pleasant.