I used to do previs just for complex effects shots. The benefits were so enormous that I now do complete 3D animatics for every job as a matter of course.
Prepping each setup in 3d doesn't just sort out what to shoot - it also nails how to shoot it. More time gets spent on the creative side of filming and less on the technical.
Previs certainly gives you significantly more bang for your budget - but there are many other advantages as well:
• Setups are explored and agreed without the pressure of a shooting schedule.
• Everyone's clear on what kind of pace and emphasis we're aiming for in the final edit.
• Shoot time is spent perfecting the action and honing performances - not searching for angles, deciding where tracks are laid or trying out lenses.
• Sets are built and dressed to the inch - with minimum wasted materials and labour.
• Previs pretty much eliminates the need for "just in case" grip or camera equipment.
• It minimises the need to shoot coverage.
• Visual effects elements slot together - so the post schedule isn't used up on fixing things.
• Animal trainers love this kind of detailed planning. It really helps keep things nice and calm - a vital part of getting good performances from animals. (And humans for that matter.)
Previs doesn't preclude spontaneity - if a better shot suggests itself on the day then we simply go for it. Nothing is lost.
The joy of previs.