When you are visiting a country where the languages are different and even the alphabets are unfamiliar, you often feel the need for a tour guide. They can be invaluable or their services can be very problematic.
Tour guiding, as well as the choice of sites you see, is often governed by politics. Tour guides are the interpreters of reality in the country you are visiting by the way they answer questions or don't answer them, use certain vocabulary, and steer you to certain places. This is rarely the whole of reality and may be a conscious or unconscious biased interpretation of reality. You may not need a tour guide for the whole trip but only for parts of it.
You are paying the guide (even if through a travel agent) and not vice versa. She or he will get $200 to $250 a day plus kick-backs from restaurants and stores and tips. You have a right to expect the guide to follow your schedule. If changes are made by the guide, examine the consequences carefully. Watch out for unscheduled stops at tourist stores. These stores are often overpriced and sell poor quality goods and the craft workers do not make a proper wage. They also take time away from your own interests. The tour guide usually gets 30% or more of the price you pay back from the store owner at these shops.
Sometimes tour guides may suggest that certain places are not safe. It may be true or it may not be. Check this kind of information out with your other local contacts. Sometimes the guide or bus driver just doesn't want to go where you want to go, and sometimes they just want to get home early. Some Israeli guides do not want you to come in contact with Palestinian Christians on a personal basis. Local contacts from NGO groups and other such organizations can tell you about safety factors.