Is that what the salesman told you? And did he get you to buy a can of Spare Air with the knife you haven't needed in over 500 dives?
My guess is people who actually need underwater cutting tools for recreational dives are probably told so for liability sake. Thus far I have never had a dive op, divemaster, or even my LDS tell me I needed a cutting tool for wimpy recreational dives that they booked or performed for me. It has also never been on a "what to bring" list.
Try diving in high current, low visibility areas where there are lots of crab and lobster pots along with monofilament around wrecks.
Its most definitely a safety issue.
Yes for 99.9% of my dives i dont need to cut anything at all - however i carry the cutting implements for that 1 dive where i do need to free myself.