They're going to earn more flights by applying for multiple different cards, e.g, three to four every 4 months, and taking advantage of the sign up bonuses. Instead of runnng $2-$3K per month through one card, they can use that money to meet the minimum spend requirements on the various card offers. That is the way for them to maximize their value and flight redemptions. They can be less agressive in the amount of applications if they want, but they will earn more by taking advantage of bonuses than they are ever going to earn by getting one card and putting all money through it.
Read the travel blogs. thepointsguy.com, frugaltravelguy.com, millionmilesecrets.com, boardingarea.com/blogs/onemileatatime, boardingarea.com/blogs/viewfromthewing, boardingarea.com/blogs/mommypoints are some of my favorites.
I know frugaltravelguy and viewfromthewing have some posts that are fairly recent that give outlines of which cards they would apply for if this was their first time taking advantage of card bonuses. Here's one such post where he lists some card for people that haven't been churning for a while. It's a little dated, so you may have to research if all of these offers are still good, but it should give you an idea. http://www.frugaltravelguy.com/2012/02/ricks-limit
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