I agree. When you have a chance to win a trophy you try to win the trophy. Especially if you've made the championship game. This isn't a best of seven or something where they are going to have to win several games to win it. It's a one off.
To quote our pal Herm Edwards, "you play to win the game."
As for EPL teams in the FA Cup, the big clubs frequently use the early round games to play their bench. Many of them play their backup goalies in Cup competitions, especially in the early rounds but sometimes all the way through. But by the time they get towards the end of the competition and the chance to win the trophy is right there in front of them they all take Herm's advice and they play to win the game.
Can they beat Houston at home without Messi? They would be favored without him. I bring him on in the 60th only if its tied. He absolutely needs to play on the weekend vs NYCFC. That is basically a playoff game there. And I'm not buying that his body wont last until December? Really. Its a couple more months and then he gets a 3 month break. He isnt elderly.