I consider a deployant clasp primarily a cost saving device, with additional benefits in safety, scratch prevention and ease of use of the watch. In my collection, all my watches with leather straps (except my Panerai) have deployant clasps on.
Leather straps (IWC or aftermarket) are not exactly cheap. Deployant clasp will significantly prolong the life of the strap. After a couple of strap replacements, you have essentially spent the same amount of money as you would on a deployant.
The only times when I see a deployant clasp is not necessary is strap is made of rubber, or strap is so thick (common on Panerai watches), a deployant clasp would not work.
Also, changing strap adds risks / opportunities to scratch your watch when the spring bars are being removed and reinstalled. With less frequent strap change, a deployant clasp will help prevent that too.
Blue