If you want an attached mag carrier, you can look to Kevin's Pocket Defender Plus. It is pictured in the ad that rotates across the top of the forum, or you can go to
www.kdholsters.com. It will be a pocketfull -- but will certainly work.
The "other reasons" I think RJ is referring to is that carrying a spare mag in your strong side pocket is not ideal. Carrying on your weak side would be better. To reload with the mag in your strong side pocket requires you to shift the gun to the other hand to dig the mag out, which is not ideal from a tactical perspective.
However, not everyone will dedicate two pockets to concealed carry and figure a spare mag in the strong side pocket is better than none at all. For those people -- a built-in mag carrier is a good solution.
I personally prefer the spare mag in the strong side pocket, as RJ suggests.
BTW -- nice holster, R.J.