JohnK would fill in here, but my understanding on the major benefits of the vertical center channel are the following:
(1) Horizontal dispersion. A bookshelf or floorstanding speaker do not suffer from the horizontal lobing effects inherent in a horizontal multi-driver speaker, providing flatter, wider horisontal dispersion.
(2) As the flip side of (1), a vertical speaker does not produce vertical dispersion as wide as a horizontal center channel, which can be beneficial because it may reduce the undesitable direct reflection from the floor in front of the speaker or from the ceiling.
(3) If you choose as the center channel a model identical to the L/R mains, that will provide a "perfect" timbre match among the L/C/R channels. A highly desirable factor.
In return, with a vertical center channel, you cannot place all drivers as close to the screen as a horizontal can. Also, in many cases especially with a big-box TV, it is difficult to install a vertical center channel in the first place. However, the latter is often a non-issue with a wall-hung plasma or
front projection.
