A) it's laid out horizontally for easy placement
B) sometimes (as in Axiom's case) they are more magnetically shielded than speakers that are not specifically marketed for that purpose (someone please correct me if this is no longer the case for Axioms!)

In other words, anything'll do for a center if you can fit it in your space and don't need to worry about magnetic fields (for example, if you have a front projection or LCD screen).

I am the Doctor, and THIS... is my SPOON!