Reusing modes can actually be quite good if you're concerned about space efficiency.
For example, having excessive controls in the car stereo might be a bad idea since drivers could potentially be distracted. So having the big volume knob to fine tune the bass level for example would be much more preferable to having six individual knobs that the user would have to look through and find.
But you only learn the interface once (and that doesn't have to happen while driving, either); then you use it again and again. So I would argue making it slightly harder to learn, but better to use in the long run (in the cases where you cannot have it both ways), makes the difference between a tool built to last, and a product made to sell.
Then there is the fact that if you only have one knob, you still have to learn how to put it in the mode you want (via secondary buttons, or even worse, just one you have to keep pressing); you gained nothing.
Also, compare typing an a cell phone "keyboard" and a PC keyboard. While you wouldn't want to have a single button for every little function (like upper-case letters), it generally makes for better usability to have a dedicated button for most of them, because while I press one key, my other fingers can already get into position to press the next in sequence.
For example, having excessive controls in the car stereo might be a bad idea since drivers could potentially be distracted. So having the big volume knob to fine tune the bass level for example would be much more preferable to having six individual knobs that the user would have to look through and find.