Justin,
Shuttles can work and be econmically feasible at the same time. One employee might equal one to one or two green fees and the cost of leasing the vehicle is very little, $5.00 per day at most.
Dan,
A lot depends on the length of the walk. Once a walking 4some leaves a green the trailing group, in carts, won't even reach that green for about 5 minutes, and that's if all goes well. They'll be chipping and putting on that green for another 5 minutes before they start on their way to the next hole, plus, it will take them at least 1 minute to travel 1/4 mile in their carts.
The walkers could have 12 minutes or so to reach the next tee and hit their tee shots, so a walk of around 1/2 mile would mean that the riders catch them on the next tee. If the groups are walker, walker, rider there is no problem with pace as the first group in the sequence will still be in the fairway of the 'next' hole. If the groups are rider, walker, rider there will be some problems. That's where a shuttle comes in, or better yet, the tee sheet gets properly arranged in the first place.