Instead of using a grid on some projects, try placing a subject outside of the flow of the rest of the design elements, maybe by mixing up shapes and colors. This picture shows a good example of focal point by not using a grid. The randomness of the shapes and placement create different focuses for the eye to rest on. When creating something like this, using a grid can actually take away from the focal point because the subject is no longer misplaced, it is in the grid pattern.
Sometimes a designer wants specific information to stand out among the rest. A drawback of using a grid in this case is that there is a set pattern and amount of space between design elements that do not allow independent elements to stand out; it's almost as if the viewer's eyes become accustomed to the uniformity. Their brains fill in what they think should be there as opposed to really looking.
Not all designs are structured; for example, abstract art. Free-flowing designs are subjective and exist outside of structure. The disadvantage of using a grid in this case is that the designer would be confined to the rigid structure of the grid as opposed to venturing out and taking a more organic approach. There would be no point in using a grid when creating a subjective piece; more useful tools would be the use of color and visual balance.
The most common disadvantage of designing in grid is the problem of overcrowding. When the designer uses a grid, he may get caught up in the idea of keeping everything "in the box" as opposed to taking advantage of white space breathing room. This can cause the viewer to feel constricted and overwhelmed by the design. It is important that the designer take a step back from the rigid structure of the grid and allow all the design elements to have their own space within the design.