I started out doing DCC so it's not really a pain for me to outfit my fleet. It's rather simple for me, buy locomotive, buy decoder, install, done! Hard wiring is not difficult at all.
Locomotives are DCC ready, why? It makes installing a decoder easier, without the costs to the manufacturer to actually put a decoder in. Atlas has tried and failed. Their excuse was that people wanted to use their favorite brands; which is kind of true. Although I will use any brand of decoders, some people swear by Digitrax, NCE, or TCS. I tend to use Digitrax decoders because they are less expensive and do what I need them to do.
It also satisfies the people who don't use DCC. Even if the locos have factory installed DCC, DCC decoders are "backwards compatible" with DC. DCC locomotives will also operate on DC.
By making locos DCC ready with an 8-pin or 9-pin plug (some of Athearn's boards can have both), the manufacturer can get away with catering to both crowds while saving money.
Eric from Boston, MA. Modeling Norfolk Southern and Pan Am Railways.