There is a small scale and large scale aspect to this issue. Ill use New Zealand as an example to illustrate the extremes.
On a large scale, you can take a 2 week organized tour of say, New Zealand. you are herded on to a charter bus, which stops at predetermined locations, you all stay in the same hotel, and all the arrangements from lodging to entrance fees to transportation is taken care of. You meet lots of new people. You don't have to worry about any planning or reservations. You have structure and a full itinerary and don't have to worry about transportation.
On a small scale, you can take a guided canyoning tour of a remote gorge in the NZ Alps. You get an expert on ropes and belays, equipment and permission to enter the gorge. You get natural history from an expert, one on one time with a local who gives you a personal perspective on their country.
You can easily organize your own large scale tour of a country, but you cant exactly go canyoning on your own. Sure if you are experienced and can rent equipment and know a gorge you can... but the core issue here is local knowledge, local access and supporting the local economy by putting money into locals hands and promoting individuals to become guides.
I think the more remote a place is, the more you should lean towards getting a guide. I had my Torres del Paine trip all planned out, but when I got there I saw guided tours with a wealth of options I never even considered. I could have had all my gear transported for me, side trips to glaciers few gringos rarely see, and even squeezed in a trip to Perito Moreno.
One more example can be found in Chile as well, in the desert North. Its high and dry and the roads are dirt. Its fairly easy to figure out the national parks and develop a kick ass itinerary- so all you need to do is rent a car. A situation developed for my friends Ruth and Barry recently when they decided to climb Cerro Toco (5,640m): It takes your breath away: adventures in the Atacama Desert
they could have paid a guide, instead they chose to rent their own truck and make the ascent on their own. Since they are serious trekkers in great shape, they had no problems making the summit and no doubt had a fantastic time. But perhaps if they paid the exorbitant price for a guide their experience would have been different. The guides might have given them 100 million years of geologic history, pointed out rare and unusual plants, made points about cultural history of the area or perhaps even stopped at a locals house for some tea.
My opinion is large scale tours are a waste of time for anyone under age 40, anyone without a family, or anyone who is not disabled. Planning your own adventure and researching a place is half the fun! However- when it comes to small scale tours of places you may or may not be able to visit on your own-local tour guides are like a golden ticket, a wealth of knowledge to experiences far beyond the reach of independent travellers reading history from a book and not meeting locals.