Bingo! This is actually a very true statement that I argue about with friends all the time. Seriously, cash payers are paying a "premium" by not taking advantage of the 2-5% premium that most prices already include due to the widespread use of CCs. There are still places out there that either won't accept CCs or have a higher rate (coincidentally about 5%) more for using CCs.
I was not a big fan of Ramsey either, but I actually think his real point is that you will spend more through plastic because it is easier to use than cash. I have tried it, and I think it is true. If I had to unfurl crinkled dollar bills for every beer at the bar, snack at the grocery store, or meal at the restaurant, it would have a much greater sting and I think I (and most people) will pay less. I don't think Ramsey ever actually argues that you will eventually pay interest/fees even if you are paying the card in full each month. His primary point, I believe, is to live beneath your means. Using plastic us just so much easier that people spend more than they would had they used cash.
I am sure excessive lines of credit is bad, but I would imagine the overall amount of credit available to you is one of the biggest factors, plus the amount of credit that you are using. I can see my credit score at all times through this identity guard thing I get through Costo. All 3 of my credit scores are regularly above 800 despite that fact that I have like 10 open credit cards. I notice that when I sign up for a new one, or if any one approaches about 75% of its limit, my credit scores dip below 800 for a month or two.
GREAT question. I hadn't researched that exact thing, but I don't think so. I found out the hard way that when using my AMEX blue for my 6% cash back, I don't get shit when I buy groceries at Wally World, Target, Costco, or any other places listed as "discounters." Basically, if you pay full price for groceries at regular name-brand grocery stores, it should show up in AMEX as eligible for the 6% cash back. Other than that, you won't get 6% for groceries bought at convienece stores, Walmart, or other places. Be wary.