My sister does this sort of thing as her "hobby". It takes a lot of time - she monitors for exceptional credit card deals and churns like crazy (which is not a problem for people on very solid financial footings, but if you are not solid financially then this sort of thing might not be for you!), monitors for shopping deals that get her lots of extra points, monitors for the lower-point level ff seat/hotel room availability, takes random flights in her free time that are cheap but get her lots of "butt in seat" miles to get higher levels of status with the airlines, and also does the same with hotel stays. Plus I'm sure she does also stuff I don't even know about. She's an attorney with her own practice, so I have no clue how she finds the time, but it's something she's passionate about so she does! (No kids helps

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I am thankful I get to be the recipient of all her hard work on occasion - she has been covering most hotel and airfare for a vacation every two years with her, me, DD14, my sister-in-law, and my two nephews. We did China two years ago, and Paris this past summer. While there are still plenty of other travel expenses I have to cover (passports, meals, activities, etc.) we definitely would not have been able to take the trips without her help. But there is no way I would have the patience or the time to do all the wheeling and dealing to make it happen - I really can't undersell the amount of work she puts in to make this stuff happen.
ETA: I myself do participate in a fair amount of credit card churning, although I typically do so for other purposes (free baggage allowances on trips, free early boarding, monetary bonuses, etc). My credit score is outstanding, and takes only a few point hit for a few months whenever I get a new card or close an old one - which is the result of a hard inquiry on my report, not really the fact that I have yet another card. And I don't see how taking advantage of an offer a company makes to me for a sign-up bonus is "theft". My credit history clearly shows I churn cards, and they still make me the offers - in fact, the more I do so, the more/better offers I get! So clearly they think there is something in it for them.
I have 7 major credit cards in my wallet right now, and another in the mail on its way to me! Most I have gotten in the last few years, one or two I've had longer term. I will be closing one or two in the next year as the due date for an annual fee comes up that I don't want to pay. But I'm sure I will find others that have nice sign-up bonuses during that same time, so I will get those. None of them have a balance, in fact, most of them are rarely even used after I've qualified for the sign-up bonus (which I don't spend extra to achieve - the ones I apply for I can usually hit with just my normal spending - I just put as many of my normal expenses as I can on them, even things like my car registration or insurance which make a big splash). Even with all those credit cards with a ridiculous amount of open credit, my recent home refinance was a walk in the park because my utilization percentage was low, low, low due to paying off everything each month. The number of credit cards are generally not an issue - it's how you USE them, what the rest of your credit history looks like and what your debt to income ratio is.