Not bad to run AC in your tank once in awhile, but only for short term. contrary to myth, AC runs out of steam in days/weeks vs months, and after that just acts as a bio filter. Use it for a week, pull it, dump it.
Water clarity is extremely important for light penetration in reefs, and our cleanest reef tanks dont compare to the quality of water in the ocean. Carbon helps a great deal in a reef. I run carbon continuosly, but change it out weekly. Intermittent use of carbon can be shocking to your system.