How is it logical to buy large high priced items on credit when you have the cash?
You just explained why and how its logical to buy large high priced items, its even more logical to buy those items on credit than the "stuff that only amounts to a few hundred a month" because the rewards and credit building will be greater.
I ran up $3,000 on my card of car parts, alone, the other month. What happened? The credit card company bumped my credit limit, it probably moved myself into a new credit rating bracket and I made extra cash out of the deal.
It doesn't matter if its $3,000 or $22,000.
If he wants to buy a "beater" of a Civic Si and he has $20,000 in the bank, he can put it on his card and invest that $20,000 for a month and make his money "work for him". He'll make money by leaving that cash in the bank for an extra month and he'll make money from the rewards that that large purchase on credit brings him.
Wow, look at that. He just made money from both aspects of the deal, while helping to build his credit.
Where's the downside?