Should a person use different window treatments, or window treatment styles in the same room? The answer is: Quite possibly. Sometimes. Depends on the space.
Mixing window treatments can be done well, if it "makes sense". For instance, in the blue and white room shown here, the long valance used over the window would not have been functional above the french doors, so simple panels were used. Mixing window treatments makes sense here -- the decorator was working with the room's design, rather than against it.
Here, again, the windows flanking the fireplace received a slightly different treatment than the adjacent bay windows. There is a reason to use different treatments -- in this case,it is to open up the light and the views and because the bay is an important architectural feature in the space.
In the very beachy and relaxing room below, the decorator used two different types of window treatments, as well as two different fabric patterns. By keeping fabric colors the same, and hanging the treatments at the same height, the designer creates unity; while introducing variety into the space with the floral pattern on the french doors. If the floral had been used on all the windows, the space might have felt a bit "precious", but using the stripe on every window might have felt a bit boring.
This image is from a Freshome article found via IBTimes.
My question for you is this: Would you be open to trying different window treatment styles, patterns, or materials in the same room? You've heard what I think -- now I'd love to hear what you think!