Ok. I resemble this remark. It is very difficult figuring out when to use DH's platinum card, but I have to put spend on it to keep the RATs at bay.
So yes, the Platinum card is hard to keep using after you've met the MSR and gotten the big SUB. Exactly what the RATs are looking for! Appeasing the RATs really leads to some suboptimal behavior.
What I do use it for is flights and award fees at 5x MR, when I can be pretty sure I won't need the
trip insurance provided by the CSR.
I'll use it for expensive purchases, because Amex provides excellent purchase protection and extended warranty coverage. But if you're counting on this coverage, then you shouldn't cancel your card even when the AF rolls around because you'll terminate the coverage when you close the card. I've made it a point to try to keep many of Amex's cards open for more than a year because the pop up mentions closing cards as a factor in your "history," and Amex gives out some pretty nice retention offers that help to offset a second year AF.
I use the Platinum for Amex Offers, and will purposely add an offer to the Plat to get some spend on the card.
When I use my Platinum, I know I'm earning only 1x MR in almost all circumstances. What else could I be earning on non-category spend? The CIU/CFU earns 1.5x UR; the BBP earns 2x MR; the SPG cards earn 2x Marriott. Of course, there are several other 2% back cards (e.g., Citi Double Cash). I'm willing to forgo a 1.5-2x base earn rate on "everything else" to put the charge on the Platinum, even if it earns only 1x MR, to appease the RATs. Any other spend that can earn me 3x or better, I'll go with the card that offers me the better category bonus.
Relatedly, I have my Platinums added to my MPX wallet and will often pay with a Platinum, earning only 1x MR, but purchase an eGC that'll earn me another 0.5-5x (or more) United miles, so it's not entirely 1x spend. When I'm buying an eGC on MPX to a merchant with a MCC that might pass-through and trigger a 3x+ category bonus on one of my Chase cards, I'll go with the Chase card. Otherwise, I'll alternate between my Platinums and SPG cards, depending on which cards need to see some spend.
Note that using the Platinum to buy eGCs negates the card's purchase protection and extended warranty benefits that would've covered your purchase. You're using the Platinum to buy an eGC, not an item so the coverage doesn't extend to the ultimate purchase. When I'm buying something expensive, I put it directly on the card and avoid paying with GCs.
I'm still breaking in my BBP, but I carry it with me all the time as my "everything else" card. Metal cards are nice, but they're heavy and thick on the wallet.