Funny you should bring it up...
I was in two gun stores the other day and saw two people buy the xd over the other brand based on the accessories that came with it.
The XD was priced the same as a glock in one store and the customer was sold on the xd based on the holster and mag carrier. FTR, he liked them equally.
In the other store, the customer was choosing between a S&W M&P9 and went with the XD. Again, the holster and mag carrier just made it easier for the clerk to sell. The customer in that case liked the way the M&P felt more, but all that "cool stuff" with big XD's all over it calls to an impulse buyer.
Both of these stores were big box retailers. The sales people at the counter did their best to push the glock, xd, and m&p equally. The XD won out both times just because it had more crap in the box.
S&W has the rebate/ mail in for two free mag thing, but as a consumer I'm pretty tired of the whole "mail in for your free whatever" campaign. The consumer electronic stores have been running that one for over 5 years now.
S&W and Glock should both pay close attention to the Springfield Armory marketing machine. Put extras in the box and make it easy for the counter jockey to sell.
OTOH the marketing is the primary reason "I" don't care for the XD's.