Re "cons", the lateral Woofer-Tweeter-Woofer design doesn't allow for ideal dispersion (it's a design compromise due to the center speaker vying for space with a TV screen). However, if you have a couple of matching center speakers, you can simply turn them sidewise and eliminate such so-called "lobing" effects (giving you a vertical w-t-w layout).
Interesting. I run Klipsch SS-1's for surrounds, which just happen to use the woofer-tweeter-woofer config that Rich refers to. So, in my particular situation, what would be best, using another SS-1, an SC-1 front center, or a bookshelf type speaker for my rear center? I've been doing searches on this topic and can't find a quick answer.