The purpose of the card is not as a One Card To Rule Them All, it's to optimize on one category of spending. As always, if you are looking for a One Card to Rule Them All, get a high-yield flat cash card like the Alliant Signature and don't look back. This card is for intermediate to advanced players who are heavily into categorizing purchases based on location. For me, I use BofA Red for online purchases, Ink Cash for cell phone/internet/office store purchases, Chase Freedom and Discover for overflow if the 5% categories are useful, BCP for supermarkets and gas, and BofA Silver for flat cash if I can't get bonused spend anywhere else. Effectively I get 5%+ back on pretty much every purchase I make outside of the things that are effectively impossible to get bonused spend on like insurance and medical bills (I get 2.62% on those.)
As for "Why would you get it over the BCP?", very simple: (1) the BCP's AF eats up a significant amount of the cashback savings (if you perfectly optimize it and spend exactly $6000, your effective cashback rate is something like 4.5%; any amount over/under this lowers the effective rate), (2) the BCP is capped just like the Custom Cash ($500/mo equates to the $6000/year in 6% supermarket spending that you would get on the BCP), and (3) Amex is incredibly finicky on their definition of "supermarkets" to get the enhanced cash back, while Citi is more permissive ("groceries" includes meat stores, dairies, bakeries, etc.)
You can very easily see that the Custom Cash stays ahead of the BCP for all amounts of spend under $6000 by plotting y=0.05x against y=0.06x - 95 on a graphing calculator.
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