I have a ton of credit cards and managing them all becomes unwieldy and costly. I am thinking of cutting some down. But in terms of just spending on one card, unlikely for me - bonus category spend adds up too much.

In terms of Amex Platinum spend - if you like the perks and service, and prioritize those over earnings, then you may as well use the card for spend. Many people do. Arguably most would do better by simply also adding an Amex Green/Gold, but I understand why many don't.