Cal, I never met Purvis in person, but I met guys who trained directly under him at NASM and I'm not talking about his clients. Purvis was the inventor of the 6 steps to exercise utilized by NASM. You probably saw this guy in those bowflex commercials. He's the guy who points at the exercisers' bodyparts and says
"look at those muscles working!". Purvis left NASM to work with Bowflex. I'd suspect Purvis is promoting some Bowflex exercise if he says dips are bad for you. Purvis is known as Mr. alterior motive.