I didn't have a planner or coordinator. Our wedding was on the huge side (by my standards, at least) and I think everything went fine. Granted, my fiance was interested in helping out. We did quite a bit of it by ourselves - invitations, rather labour intensive favours, etc. The thing we needed the most help with was the decorating of the reception venue. Thankfully, our bridal party and family were absolutely fantastic and extremely helpful.
IMO, the decision on planner vs. no planner depends entirely on the couple. If they're very hands-on and decisive then a planner may create more stress. In this case, a day-of coordinator would be best. The one place a planner can be very useful is in working directly with vendors to save the couple money. I'm sure your coworker will do what's best for them.
Oh, and we went with a DJ for the exact opposite reason that you recommend against them - it was a huge savings compared to getting a band!