The lack of choice is (mostly) a bad idea.
Only giving the slider as an input method can be good for novice users and/or when accuracy is not important as there is less cognitive load. They don't have to think about the different input methods.
This starts to fail when you want to enter a specific value quickly and fails even more if the range of values is large (accuracy becomes difficult).
An input field can be quicker, particularly when using tab to advance to the next field, and more accurate.
A slider makes it easier to compare values of a similar range and to see trends in cases such as a graphic equalizer (in fact in this case it is unlikely you would want text input for each value).
It is interesting to note that MS Windows (and most media players) only gives a slider for volume control. Even more interesting is that in Windows 7 (not sure about others - this is what I am using at the moment) only tells you the actual value as a tooltip either when hovering on the volume icon or while actually dragging the slider - it is not permanently shown at all.
Photoshop has an interesting take on these types of input. A slider is used in conjunction with a text input but when the text input is focused the mouse wheel can also be used to adjust the value.