We are working on a meat packing plant that will have a 15kV lineup of outdoor metal-enclosed load interrupter switchgear. These switches will feed pad mount transformers (underground concrete encased PVC ductbank) which will serve 480/277V main-tie-main switchboards located throughout the facility. Should we provide a spare conduit for each 15 kV feeder? Pros? Cons?

I wouldn't provide one if it is not provided for in your contract. It's funny that you should ask because every medium voltage ductbank that I've ever been involved in had a spare conduit in it. That especially holds true for primary coming from the utility.
Hit the customer up for an extra. If they ever have a failure without an extra conduit to pull in a new feeder, they may regret having not provided one while the project is under construction.

Extra Change, Bling, bling,
Doc

the major cost in installing your ductbank is in the excavation and placement of concrete.An extra 5" conduit won't add to much to the cost.If the runs are long I would intall steel 90's have seen the pull rope burn through pvc more times than I would care to say