I agree with Kuki that often the ship does use "scare" tactics in order to push their excursions. I have personally experienced it with NCL in the Baltics. They published, in the daily, that you would not be allowed off the ship unless you were with one of the ship's excursions or obtained a Russian Visa prior to the cruise. I spoke to the Hotel Manager and told him that if you have an excursion with a registered tour operator the 72 hour tourist visa is obtained by them. You only need to show a valid "ticket" or confirmation with the tour operator's name and id number. His response was: "You can get off the ship but you'll never make it through immigration." We walked right through immigration with no problems as did a couple hundred other people. We booked a private guide and driver and our guide was more than irritated with the cruise lines because she had many, last minute, cancellations and no shows because of this.
Also: In China, on HAL, the similar but less Draconian, tactics were used. They also told everyone they should book a tour only with the ship because of the problems in China and the distance from Xingang to Beijing. I can somewhat see their logic, as Bruce pointed out, because the chances of delay are far greater because of the two hour distance from Xingang to Beijing. The main problem we had was obtaining our passports. The ship held them because of the PIA Chinese immigration regulations. It took us almost an hour and an act of the Hotel Manager to get our passports for our private tour. We were constantly told that a "photo copy" of our passports was all that was needed. I knew better and they should have too because the overnight ship's tour could not check in to their hotel because they did not have their passports. They sat there for over three hours while someone drove back to the ship and retrieved the passports. Also, they did not retrieve all of them and some people had to return to the ship. There were some very unhappy campers.
I also have to strongly disagree with Merced Mike based on my experiences with cruise line and private tours. RARELY have I seen an equivalent tour from the ship close in price to a private tour. As an example: The above mentioned tour in China cost us $650 (2 people) for two days, Intercontinental Hotel, all admission fees to all sites requiring fees and three meals per day. The ship's tour was over $500/pp and did far less because of having to move the number of passengers. Also, the quality of tours I have received from reputable tour providers is far better than those of most ship tours I've been on. Also, on comparing notes with other passengers we have seen more and spent more time at sites than they have.
There was one instance in Shanghai where we had a private guide. We were at the Temple of the Jade Buddha and there were a number of people from the ship who had booked a private car and guide through the ship. Our guide was describing the buddha and history of the temple and the guides from the ship were listening to him and then parroting what he said. It got to the point that they were pushing in front of us with their clients to hear him. I blew up over that and ripped one guide a new one in front of his clients. We met the people who were with him and another guide and they both apologized and were very disappointed in the quality of the guides though the guides touted they had degrees in Chinese History.
I have to also say that some of the most enjoyable tours we've had on the islands and in Europe were from simple cab drivers. Some independent research and a few minutes of speaking with a cab driver can give you a good idea if they know what they are talking about and if there is a good chance you will receive value for your money.
There is also a level of personal preference. If you like to tour with a large group then that is great. However, I want to be able to stop when I want and if there is something I see and I want to take more time then I can.
Bruce does make some very good points about the risks of independent tours and the impact it can have on the ship and other passengers. I do agree that there are people who have little travel experience who just grab the cheapest tour at the dock and end up late or missing the ship or are ripped off. I am fortunate that I can get by in a couple of languages and my wife is fluent in three others and can get by in a couple more so that helps. However I have also found that, especially in Europe, it isn't too hard to find people who speak English.
There is an old joke that has a lot of truth to it.
"What do you call someone who speaks three languages?" Multi-lingual
"What do you call someone who speaks two languages?" Bi-lingual
"What do you call someone who speaks one language? American